From e24326b735d3be785fbba7df47f36e985f8555c3 Mon Sep 17 00:00:00 2001
From: Jonas Reith <jonasreith@gmail.com>
Date: Mon, 31 Aug 2026 19:11:16 +0200
Subject: [PATCH] Add a Main Menu + save browser, replacing auto-generate and
 the one-slot save

The app used to always generate a world straight from planet.cfg on launch
and F5 always overwrote a single fixed planet.save. Now it opens on a
full-screen Main Menu (Home / Settings / Load World) so every PlanetConfig
field, the seed, starting a new world, and browsing saves are all reachable
without hand-editing planet.cfg, and saves are timestamped so nothing is
silently overwritten. A toolbar button reopens the menu later without
disturbing a running world.

The settings editor is generic (no per-field UI code): configFieldTable()
reuses the existing CONFIG_FIELDS X-macro to build a runtime field table,
grouped into 25 categories via an explicit name->category lookup (a
"first field of each section" boundary scan was tried first and is wrong,
since CONFIG_FIELDS emits all doubles, then all ints, then the seed --
not struct declaration order, so most categories aren't contiguous in that
order). Save v25 adds the viewer's colour-mode/overlay state, kept out of
Planet::writeState/readState's signature so it touches none of the
existing headless tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TMfyZv91tonDnPJqbaTVJE
---
 .gitignore                  |   2 +-
 BUILD.md                    |  45 +++---
 CLAUDE.md                   |  88 ++++++++++--
 CMakeLists.txt              |   9 ++
 src/render/MainMenu.cpp     | 275 ++++++++++++++++++++++++++++++++++++
 src/render/MainMenu.hpp     |  35 +++++
 src/render/Toolbar.cpp      |  11 +-
 src/render/Viewer.cpp       | 102 +++++++++++--
 src/render/Viewer.hpp       |  15 +-
 src/render/ViewerInput.cpp  |  10 +-
 src/render/ViewerRender.cpp |  10 ++
 src/sim/Planet.hpp          |  16 +++
 src/sim/PlanetIO.cpp        | 201 ++++++++++++++++++++++++++
 test_viewerstate.cpp        |  80 +++++++++++
 14 files changed, 854 insertions(+), 45 deletions(-)
 create mode 100644 src/render/MainMenu.cpp
 create mode 100644 src/render/MainMenu.hpp
 create mode 100644 test_viewerstate.cpp

diff --git a/.gitignore b/.gitignore
index 6a03212..f1a29a3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,7 +3,7 @@
 
 # Runtime-generated, machine-specific
 planet.cfg
-planet.save
+*.save
 screenshot*.png
 
 # Claude Code local (machine-specific) settings
diff --git a/BUILD.md b/BUILD.md
index ccd56fc..19ded52 100644
--- a/BUILD.md
+++ b/BUILD.md
@@ -72,7 +72,7 @@ the full ~2.8x speedup; the default uses all cores for no extra gain:
     F           fast-forward Phase-1 forming to settled (instant)
     R           reseed planet (restart forming)
     + / -       subdivision level (detail), 1..7
-    F5 / F9     save / load full state (planet.save)
+    F5 / F9     save / load full state (writes/reads a timestamped world_<time>.save)
     F12         screenshot to screenshot.png
     F11         export the 2D map as a high-resolution (~4K) PNG (also a toolbar button)
     Shift+F11   export a labeled atlas (~8K, every place/settlement name, no overlap; toolbar button)
@@ -80,28 +80,39 @@ the full ~2.8x speedup; the default uses all cores for no extra gain:
     F1          toggle the clickable toolbar menu (top-right of the 3D globe; a button/
                 checkbox/dropdown for every key above, built with raygui)
 
-CLI flags (applied before the first load/generate):
+The app opens on a full-screen **Main Menu** (not a live world): a Home tab (New
+World / Back to World / seed + Randomize), a Settings tab editing every one of the
+~240 PlanetConfig fields (grouped into 25 categories), and a Load World tab listing
+every save. A toolbar "Main Menu" button (World/Time section) reopens it later
+without disturbing a running world.
+
+CLI flags (applied before the Main Menu is shown):
 
     --seed N        override the config seed
     --config PATH   use an alternate config file instead of planet.cfg
 
 ## Files (written in the working directory)
 
-    planet.cfg   human-editable key=value config of every PlanetConfig parameter;
-                 auto-created on first run, reload live with F2. Range-checked on
-                 load; an invalid file reverts to safe defaults (not overwritten).
-    planet.save  binary snapshot (versioned, currently v22: +diplomacy; v21
-                 +conflict/wars; v20 +settlements; v19 +ecoregions; v18
-                 +geography salt; v17 +geography/atlas; v16 +event log; v15
-                 +stateful volcanoes; v13 +Live World clock rate; v12
-                 +step-back history (~40 frames, so a load can rewind storms);
-                 v11 +weather systems/storms; v10 +weather fields; v9 +moons;
-                 v8 +Live World clock; v7 +biota): seed + config + full planet state; F5
-                 writes it, F9 reloads and resumes deterministically. As of v6
-                 the config is stored as a self-describing key=value block (like
-                 planet.cfg), so adding/removing config fields no longer breaks saves
-                 (unknown keys ignored, missing keys default). v6 cannot load pre-v6
-                 saves (one-time break) -- regenerate them.
+    planet.cfg          human-editable key=value config of every PlanetConfig
+                         parameter; auto-created on first run, reload live with F2
+                         or edit any field in the Main Menu's Settings tab.
+                         Range-checked on load/New World; invalid settings revert
+                         to safe defaults (planet.cfg itself is not overwritten).
+    world_<time>.save   one binary snapshot per save (versioned, currently v25:
+                         +viewer colour-mode/overlay state; v24 +edit-mode locks;
+                         v23 +cultural evolution; v22 +diplomacy; v21
+                         +conflict/wars; v20 +settlements; v19 +ecoregions; v18
+                         +geography salt; v17 +geography/atlas; v16 +event log; v15
+                         +stateful volcanoes; v13 +Live World clock rate; v12
+                         +step-back history (~40 frames, so a load can rewind storms);
+                         v11 +weather systems/storms; v10 +weather fields; v9 +moons;
+                         v8 +Live World clock; v7 +biota): seed + config + full planet
+                         state. F5/"Save" always writes a fresh timestamped file;
+                         F9/"Load" (or the Main Menu's Load World tab) picks/lists
+                         them. As of v6 the config is stored as a self-describing
+                         key=value block (like planet.cfg), so adding/removing config
+                         fields no longer breaks saves (unknown keys ignored, missing
+                         keys default). v6 cannot load pre-v6 saves (one-time break).
 
 ## Config (planet.cfg, or PlanetConfig defaults in code)
 
diff --git a/CLAUDE.md b/CLAUDE.md
index c1f25ee..9ee5d67 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -501,6 +501,57 @@ on the Live World clock). **Steps 1–7 of the roadmap are done (plus a derived
   Verified live: one continent now shows six distinct peoples instead of a single dominant culture,
   with realms still correctly mono-cultural. `src/sim` + `src/render`; cultures are derived/not saved,
   so no save-format version bump.
+- **Main Menu + a proper save browser (was: always auto-generate, one fixed save slot)** *(done — see
+  `src/render/MainMenu.{hpp,cpp}`, `Viewer::startNewWorld()`, save v25)* — the app used to always
+  generate a world straight from `planet.cfg` on launch, and `F5` always overwrote the same
+  `planet.save`. Now it opens on a full-screen **Main Menu** (`AppScreen::MainMenu`, a complete early
+  return in `renderFrame()`/`handleInput()` — not a dim overlay like the Phase-3 modal, since the menu
+  must render before `planet.generate()` has ever run): a **Home** tab (New World / Back to World —
+  the latter only once a world exists — a prominent Seed field + Randomize, and status text), a
+  **Settings** tab editing **every one of the ~240 `PlanetConfig` fields**, and a **Load World** tab
+  listing every `*.save` file. Reachable again later via a toolbar "Main Menu" button (World/Time
+  section) without disturbing a running world — "Back to World" is a pure screen-flag flip. **Saves
+  are now timestamped**: `F5`/"Save" always writes a fresh `world_<timestamp>.save`
+  (`makeTimestampedSavePath()`, the same `strftime` idiom as the map/atlas export); `F9`/"Load" picks
+  the newest via `listSaveFiles()` (`LoadDirectoryFilesEx(".", ".save", false)`); the fixed
+  `SAVE_PATH` member is gone. **A generic settings editor drives all 240 fields with no per-field
+  code**: `Planet::configFieldTable()` (`src/sim/PlanetIO.cpp`) reuses the existing
+  `CONFIG_FIELDS(D, I, U)` X-macro a third time to build a runtime `vector<ConfigFieldDesc>` (name +
+  category + type-erased get/set lambdas) — the same "add a field, everything downstream keeps
+  working" property `writeConfigFields()`/`parseConfigStream()` already give `planet.cfg`/saves.
+  Fields are grouped into **25 categories** via an explicit `name → category` lookup table
+  (`kConfigFieldCategory`, hand-verified against `PlanetConfig`'s actual declaration order in
+  `PlanetTypes.hpp`) — **not** a "first field of each section" boundary scan over
+  `configFieldTable()`'s own iteration order, which was tried first and is a real trap: `CONFIG_FIELDS`
+  expands to all `D()` fields, then all `I()` fields, then `U(seed)`, so a category whose fields mix
+  types (nearly all of them — e.g. "Plate lifecycle" has 9 doubles early and 5 ints in the separate
+  int run) is **not contiguous** in that order; a boundary scan silently merged everything after the
+  first non-contiguous category into one bucket. `configCategoryList()` returns the 25 labels in
+  display order, derived from `kConfigFieldCategory`'s own (genuinely contiguous, hand-written) block
+  order. The Settings tab is a left category rail + a `GuiScrollPanel` field list (the exact
+  scroll/scissor pattern from `Toolbar.cpp`); each row is a generic `drawConfigFieldRow()` modeled on
+  Edit Mode's `editRow()` (`Panels.cpp`) minus the lock column, and `handleMainMenuInput()`'s
+  click-to-focus/type/Enter-commits/Escape-cancels loop mirrors Edit Mode's typed-numeric-entry
+  handling (`ViewerInput.cpp`) exactly — click a field, type a new value, Enter calls
+  `Viewer::commitMenuField()` (a `std::stod` in a `try/catch`, silently discarding on a bad number,
+  same as `commitEditField()`). No live bounds-checking while typing: `validateConfig()` only runs at
+  "New World" time (same as the existing `F2` behaviour), reverting to defaults on failure. **Save v25**
+  appends the viewer's **colour mode + all overlay toggles** (`showBorders`/`showDrift`/.../
+  `showRealms`, 13 bits) so a loaded world shows what it looked like when saved, not just its
+  simulation state — deliberately kept **out of** `Planet::writeState`/`readState`'s 15-`bool hasX`
+  signature (same idiom already used there for `settled`/`phase3`/`liveWorld`/`liveRate`) so this
+  purely-viewer-side addition touches zero of the ~18 `test_*.cpp` files that call `readState`
+  directly; instead it's written/read directly in `Viewer::saveGame()`/`loadGame()`, positioned after
+  the existing event-journal block (a live bug caught the position mismatch: the read was first
+  wired in right after the header fields while the write happens at the very end of the file, which
+  silently desynced every read after it and broke the *unrelated* `test_events` suite too — moving
+  the read to match the write's actual position fixed both). Camera angle, map zoom/pan, selected
+  cell and the Live-info tab stay session-local, matching the user's explicit scope call. New headless
+  `test_viewerstate.cpp` (modeled on `test_events.cpp`): the v25 round-trip, plus a version-patched
+  negative test confirming a pre-v25 save loads without crashing and leaves mode/overlays untouched.
+  `src/sim` (`Planet.hpp`, `PlanetIO.cpp`) + `src/render` (`Viewer.{hpp,cpp}`, `ViewerInput.cpp`,
+  `ViewerRender.cpp`, `Toolbar.cpp`, new `MainMenu.{hpp,cpp}`); `.gitignore`'s `planet.save` line
+  became `*.save`.
 
 ## Current state
 
@@ -923,6 +974,8 @@ src/
     Picking.*         mouse ray / sphere hit / nearest-cell / angle helpers
     Panels.*          right-column UI: detail panel, hover info, world stats, edit-mode panel
     Toolbar.*         clickable raygui sidebar menu (mouse-friendly counterpart to the key list)
+    MainMenu.*        full-screen Main Menu: generic settings editor (all PlanetConfig fields), seed,
+                      New World / Load World / Back to World (AppScreen::MainMenu vs. World)
     RayGuiImpl.cpp    the one RAYGUI_IMPLEMENTATION translation unit (raygui.h is declaration-only elsewhere)
     Viewer.{hpp,cpp}  Viewer struct: all state + setup + sim orchestration
     ViewerInput.cpp   handleInput(): camera, hover picking, click, keys
@@ -1046,7 +1099,10 @@ on a settled world; re-press regenerates) · `W` enter/leave **Live World** (set
 settlement — with per-field lock against automatic recompute; see below) ·
 `F1` toggle the clickable **toolbar menu** (top-right of the 3D globe; every key above also has a
 button/checkbox/dropdown there — see below) ·
-`+`/`-` subdivision level (1..7) · `F5` save (`planet.save`) · `F9` load ·
+`+`/`-` subdivision level (1..7) · `F5` save (writes a fresh `world_<timestamp>.save`) · `F9` load
+(the newest save) · a toolbar "Main Menu" button (World/Time section) opens the full-screen **Main
+Menu** — edit any of the ~240 `planet.cfg` fields, set a seed, start a New World or browse/Load
+World — without disturbing a running world (see below) ·
 `F12` screenshot (`screenshot.png`) · `F11` export the 2D map as a high-resolution (~4K)
 `map_export_<timestamp>.png` (also a toolbar button) · `Shift+F11` export a labeled reference atlas
 (~8K, every geography feature + settlement name, no overlapping text — also a toolbar button) ·
@@ -1101,15 +1157,20 @@ Mode. Locked out (`GuiDisable`d) while the Phase-3 "start hydrology?" prompt is
 everything else.
 
 CLI: `--seed N` overrides `cfg.seed`; `--config PATH` uses an alternate config
-file (both applied before the initial load/generate).
+file (both applied before the initial load/generate). The app opens on the **Main
+Menu** rather than auto-generating a world; `planet.generate()` runs only once you
+pick New World or Load World there (see the Main Menu entry above).
 
-Two files in the working dir: `planet.cfg` (human-editable key=value of every
-PlanetConfig param, auto-created on first run, reload with `F2`) and
-`planet.save` (binary: seed + config + full planet state, written/read by
-`Planet::writeState`/`readState`, resumes deterministically). Config is
-range-checked by `validateConfig()` on load/`F2`; an invalid file reverts to safe
-defaults (without overwriting your `planet.cfg`) and shows a status message. The
-save header is versioned (currently **24**; v2 adds the `[`/`]` drift rate, v3 a
+`planet.cfg` (human-editable key=value of every PlanetConfig param, auto-created
+on first run, reload with `F2` or edit any field live in the Main Menu's Settings
+tab) plus **one `world_<timestamp>.save` file per save** in the working dir
+(binary: seed + config + full planet state, written/read by
+`Planet::writeState`/`readState`, resumes deterministically) — `F5`/the toolbar
+"Save" button always write a fresh timestamped file, `F9`/"Load" (or the Main
+Menu's Load World tab) picks/lists them via `listSaveFiles()`. Config is
+range-checked by `validateConfig()` on load/`F2`/New World; an invalid file/settings
+reverts to safe defaults (without overwriting your `planet.cfg`) and shows a status
+message. The save header is versioned (currently **25**; v2 adds the `[`/`]` drift rate, v3 a
 `phase3` flag, v4 a per-cell biome byte, v6 stores config as a **self-describing
 key=value text block** instead of a raw POD dump, v7 appends the **biota population**
 block — three Organism lists per cell, gated by a flag byte, v8 appends the **Live World**
@@ -1130,7 +1191,10 @@ counter (cultural evolution), likewise per-frame (per-settlement vector + the li
 truncates, a replay re-creates), and v24 the **edit mode** block — a per-cell `editLock` bitmask
 (manual-edit locks, `F3`) as a trailing vector plus sparse locked-value maps for the climate/biota-
 density/habitability fields (which have no other persistent storage — the map only holds an entry
-while its lock bit is set);
+while its lock bit is set), and v25 the viewer's **colour mode + overlay toggles**
+(kept out of `Planet::writeState`/`readState`'s signature — written/read directly in
+`Viewer::saveGame()`/`loadGame()`, so it doesn't touch the ~18 `test_*.cpp` files that
+call `readState` directly);
 newer-than-supported is
 rejected. Older saves (no biota block) load fine with an empty population (press `L`);
 pre-v8 saves load with Live World off; pre-v9 saves synthesize moons from the seed; pre-v10
@@ -1143,7 +1207,9 @@ demand via `M`); pre-v19 saves load with no ecoregions (regenerated via `E`); pr
 no settlements (re-seeded via `U`); pre-v21 saves load with no wars (everyone independent; wars begin
 again as the clock runs); pre-v22 saves load with no diplomacy (relations re-form as the clock runs);
 pre-v23 saves load with no culture state (re-seeded one-per-continent on the next refresh — the same
-peoples as before; evolution starts from there); pre-v24 saves load with no manual edits/locks.
+peoples as before; evolution starts from there); pre-v24 saves load with no manual edits/locks;
+pre-v25 saves load with the colour mode/overlays left at whatever the Viewer already held (same
+"session state survives a load" behaviour saves have always had for anything not in the file).
 A load drops any **stale** pre-load `wxUndo` history and reloads the
 saved one.
 **As of v6, adding/removing PlanetConfig fields no longer breaks saves** — the saved
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f1a3477..fac4276 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -63,6 +63,7 @@ set(RENDER_SOURCES
     src/render/Picking.cpp
     src/render/Panels.cpp
     src/render/Toolbar.cpp
+    src/render/MainMenu.cpp
     src/render/RayGuiImpl.cpp
     src/render/Viewer.cpp
     src/render/ViewerInput.cpp
@@ -106,3 +107,11 @@ if(UNIX AND NOT APPLE)
     target_link_libraries(test_events PRIVATE m pthread dl)
 endif()
 add_test(NAME events COMMAND test_events)
+
+add_executable(test_viewerstate test_viewerstate.cpp ${RENDER_SOURCES})
+target_include_directories(test_viewerstate PRIVATE src/sim src/render ${raygui_SOURCE_DIR}/src)
+target_link_libraries(test_viewerstate PRIVATE planetsim_sim raylib)
+if(UNIX AND NOT APPLE)
+    target_link_libraries(test_viewerstate PRIVATE m pthread dl)
+endif()
+add_test(NAME viewerstate COMMAND test_viewerstate)
diff --git a/src/render/MainMenu.cpp b/src/render/MainMenu.cpp
new file mode 100644
index 0000000..64f4e4d
--- /dev/null
+++ b/src/render/MainMenu.cpp
@@ -0,0 +1,275 @@
+#include "MainMenu.hpp"
+#include "Viewer.hpp"
+#include "raygui.h"
+#include <cstring>
+
+// The full-screen Main Menu (see MainMenu.hpp). Layout is a pure function of screenW/screenH (no
+// world needs to exist), so geometry helpers here are shared verbatim between drawMainMenu() (which
+// rebuilds the click-target rect lists in MainMenuState every frame -- same idiom as
+// Panels.cpp's edit panel) and handleMainMenuInput() (which reads them back). Field editing mirrors
+// Edit Mode's click-to-type interaction (Panels.cpp::editRow / ViewerInput.cpp's typing loop /
+// Viewer::commitEditField) but is driven generically by configFieldTable() instead of a hand-written
+// row per field.
+namespace {
+    const float HEADER_H = 64.0f;
+    const float TAB_W = 160.0f, TAB_H = 32.0f, TAB_GAP = 8.0f, TAB_X0 = 260.0f;
+
+    struct TabDef { MenuTab tab; const char* label; };
+    const TabDef kTabs[] = {
+        { MenuTab::Home,      "Home" },
+        { MenuTab::Settings,  "Settings" },
+        { MenuTab::LoadWorld, "Load World" },
+    };
+
+    Rectangle tabButtonRect(int i) {
+        return Rectangle{ TAB_X0 + i * (TAB_W + TAB_GAP), 16.0f, TAB_W, TAB_H };
+    }
+
+    Rectangle mainMenuBody(const Viewer& v) {
+        return Rectangle{ 0.0f, HEADER_H, (float)v.screenW, (float)v.screenH - HEADER_H };
+    }
+
+    // configFieldTable() index of the "seed" field (U(seed), the CONFIG_FIELDS token name),
+    // resolved once and shown prominently on the Home tab.
+    int seedFieldIndex() {
+        static const int idx = [] {
+            const auto& t = configFieldTable();
+            for (size_t i = 0; i < t.size(); ++i)
+                if (std::strcmp(t[i].name, "seed") == 0) return (int)i;
+            return -1;
+        }();
+        return idx;
+    }
+
+    // Draw one editable config-field row: a label + a value box (click to type, like Edit Mode's
+    // editRow() in Panels.cpp, minus the lock column -- config fields have no lock concept).
+    // Records the row's hit rect + table index into m.fieldRects/fieldIndices for the click handler.
+    int drawConfigFieldRow(MainMenuState& m, int fieldIdx, const ConfigFieldDesc& d, double value,
+                           int x, int y, int w) {
+        Rectangle r{ (float)x, (float)y, (float)w, 22.0f };
+        bool active = (m.editingConfigField == fieldIdx);
+        DrawRectangleRec(r, active ? Color{40, 46, 64, 255} : Color{22, 24, 34, 255});
+        DrawRectangleLinesEx(r, 1, Color{70, 74, 90, 255});
+        DrawText(d.name, (int)r.x + 4, (int)r.y + 3, 14, Color{190, 195, 210, 255});
+        std::string val = active ? (m.editBuffer + "_")
+                         : (d.isInteger ? TextFormat("%.0f", value) : TextFormat("%.4g", value));
+        int vw = MeasureText(val.c_str(), 14);
+        DrawText(val.c_str(), (int)(r.x + r.width) - vw - 6, (int)r.y + 3, 14,
+                active ? Color{255, 230, 140, 255} : RAYWHITE);
+        m.fieldRects.push_back(r);
+        m.fieldIndices.push_back(fieldIdx);
+        return y + 24;
+    }
+
+    // Fixed geometry for the Home tab's non-field buttons (New World / Back to World / Randomize)
+    // -- shared verbatim by drawHomeTab() and its click handler so they can never drift apart.
+    struct HomeLayout { Rectangle newBtn, backBtn, seedRow, randBtn; };
+    HomeLayout computeHomeLayout(Rectangle body) {
+        HomeLayout L;
+        float x = body.x + 40.0f, y = body.y + 90.0f;
+        L.newBtn  = Rectangle{ x, y, 220.0f, 44.0f };
+        L.backBtn = Rectangle{ x + 220.0f + 16.0f, y, 220.0f, 44.0f };
+        y += 44.0f + 66.0f;
+        L.seedRow = Rectangle{ x, y, 280.0f, 30.0f };
+        L.randBtn = Rectangle{ x + 280.0f + 12.0f, y, 140.0f, 30.0f };
+        return L;
+    }
+
+    void drawHomeTab(Viewer& v, Rectangle body) {
+        MainMenuState& m = v.menuState;
+        HomeLayout L = computeHomeLayout(body);
+        int x = (int)body.x + 40;
+
+        DrawText("World Creation", x, (int)body.y + 40, 26, RAYWHITE);
+
+        DrawRectangleRec(L.newBtn, Color{70, 110, 70, 255});
+        DrawRectangleLinesEx(L.newBtn, 1, Color{110, 160, 110, 255});
+        DrawText("New World", (int)L.newBtn.x + 20, (int)L.newBtn.y + 12, 18, RAYWHITE);
+
+        bool canBack = m.worldEverGenerated;
+        DrawRectangleRec(L.backBtn, canBack ? Color{60, 70, 100, 255} : Color{34, 36, 44, 255});
+        DrawRectangleLinesEx(L.backBtn, 1, canBack ? Color{100, 120, 160, 255} : Color{60, 62, 70, 255});
+        DrawText("Back to World", (int)L.backBtn.x + 16, (int)L.backBtn.y + 12, 18,
+                canBack ? RAYWHITE : Color{110, 112, 120, 255});
+
+        DrawText("Seed", x, (int)L.seedRow.y - 22, 18, Color{190, 195, 210, 255});
+        int si = seedFieldIndex();
+        if (si >= 0) {
+            const auto& t = configFieldTable();
+            double val = t[si].get(v.cfg);
+            bool active = (m.editingConfigField == si);
+            DrawRectangleRec(L.seedRow, active ? Color{40, 46, 64, 255} : Color{22, 24, 34, 255});
+            DrawRectangleLinesEx(L.seedRow, 1, Color{70, 74, 90, 255});
+            std::string vs = active ? (m.editBuffer + "_") : TextFormat("%.0f", val);
+            DrawText(vs.c_str(), (int)L.seedRow.x + 8, (int)L.seedRow.y + 6, 18,
+                    active ? Color{255, 230, 140, 255} : RAYWHITE);
+            m.fieldRects.push_back(L.seedRow);
+            m.fieldIndices.push_back(si);
+        }
+
+        DrawRectangleRec(L.randBtn, Color{50, 50, 70, 255});
+        DrawRectangleLinesEx(L.randBtn, 1, Color{90, 90, 120, 255});
+        DrawText("Randomize", (int)L.randBtn.x + 14, (int)L.randBtn.y + 6, 16, RAYWHITE);
+
+        int hy = (int)L.seedRow.y + 60;
+        DrawText("Tune every setting on the Settings tab, then start a new world here.",
+                x, hy, 16, Color{160, 164, 180, 255});
+        hy += 26;
+        DrawText("New World applies every change made here; unsaved edits to a running world",
+                x, hy, 16, Color{160, 164, 180, 255});
+        hy += 20;
+        DrawText("are kept if you just go Back to World instead.", x, hy, 16, Color{160, 164, 180, 255});
+        if (!v.statusMsg.empty() && GetTime() < v.statusUntil) {
+            hy += 30;
+            DrawText(v.statusMsg.c_str(), x, hy, 16, Color{120, 230, 140, 255});
+        }
+    }
+
+    void drawSettingsTab(Viewer& v, Rectangle body) {
+        MainMenuState& m = v.menuState;
+        const auto& cats = configCategoryList();
+        const auto& table = configFieldTable();
+
+        const float railW = 280.0f;
+        Rectangle rail{ body.x + 24.0f, body.y + 16.0f, railW, body.height - 32.0f };
+        DrawRectangleRec(rail, Color{16, 18, 26, 255});
+        DrawRectangleLinesEx(rail, 1, Color{70, 74, 90, 255});
+        float ry = rail.y + 4.0f;
+        for (size_t i = 0; i < cats.size(); ++i) {
+            Rectangle r{ rail.x + 4.0f, ry, rail.width - 8.0f, 26.0f };
+            bool active = (m.settingsCategory == (int)i);
+            DrawRectangleRec(r, active ? Color{50, 58, 84, 255} : Color{24, 26, 36, 255});
+            DrawText(cats[i], (int)r.x + 8, (int)r.y + 5, 14,
+                    active ? Color{255, 230, 140, 255} : Color{190, 195, 210, 255});
+            m.categoryRects.push_back(r);
+            ry += 28.0f;
+        }
+
+        float px = rail.x + rail.width + 24.0f;
+        Rectangle scrollBounds{ px, body.y + 16.0f, (body.x + body.width) - px - 24.0f, body.height - 32.0f };
+        const char* cat = (m.settingsCategory >= 0 && m.settingsCategory < (int)cats.size())
+                         ? cats[m.settingsCategory] : "";
+        int count = 0;
+        for (const auto& d : table) if (std::strcmp(d.category, cat) == 0) ++count;
+        Rectangle content{ 0, 0, scrollBounds.width - 16.0f, (float)count * 26.0f + 16.0f };
+        Rectangle view{};
+        GuiScrollPanel(scrollBounds, nullptr, content, &m.settingsScroll, &view);
+        BeginScissorMode((int)view.x, (int)view.y, (int)view.width, (int)view.height);
+        float fx = scrollBounds.x + 8.0f + m.settingsScroll.x;
+        float fy = scrollBounds.y + 8.0f + m.settingsScroll.y;
+        float fw = content.width - 16.0f;
+        for (size_t i = 0; i < table.size(); ++i) {
+            if (std::strcmp(table[i].category, cat) != 0) continue;
+            double val = table[i].get(v.cfg);
+            fy = (float)drawConfigFieldRow(m, (int)i, table[i], val, (int)fx, (int)fy, (int)fw);
+        }
+        EndScissorMode();
+    }
+
+    void drawLoadWorldTab(Viewer& v, Rectangle body) {
+        MainMenuState& m = v.menuState;
+        int x = (int)body.x + 40, y = (int)body.y + 30;
+        DrawText("Load World", x, y, 22, RAYWHITE);
+        y += 40;
+        if (m.saveFiles.empty()) {
+            DrawText("No saves found.", x, y, 16, Color{160, 164, 180, 255});
+            return;
+        }
+        for (const auto& f : m.saveFiles) {
+            Rectangle r{ (float)x, (float)y, 560.0f, 28.0f };
+            DrawRectangleRec(r, Color{22, 24, 34, 255});
+            DrawRectangleLinesEx(r, 1, Color{70, 74, 90, 255});
+            DrawText(f.c_str(), (int)r.x + 8, (int)r.y + 5, 16, RAYWHITE);
+            m.saveFileRects.push_back(r);
+            y += 32;
+        }
+    }
+}
+
+void drawMainMenu(Viewer& v) {
+    MainMenuState& m = v.menuState;
+    m.categoryRects.clear();
+    m.fieldRects.clear(); m.fieldIndices.clear();
+    m.saveFileRects.clear();
+
+    DrawText("Planet Sim", 16, 14, 30, RAYWHITE);
+    for (size_t i = 0; i < std::size(kTabs); ++i) {
+        Rectangle r = tabButtonRect((int)i);
+        bool active = (m.tab == kTabs[i].tab);
+        DrawRectangleRec(r, active ? Color{60, 70, 100, 255} : Color{28, 30, 42, 255});
+        DrawRectangleLinesEx(r, 1, Color{90, 90, 110, 255});
+        int tw = MeasureText(kTabs[i].label, 18);
+        DrawText(kTabs[i].label, (int)(r.x + r.width / 2 - tw / 2), (int)(r.y + 7), 18,
+                active ? RAYWHITE : Color{180, 180, 195, 255});
+    }
+
+    Rectangle body = mainMenuBody(v);
+    switch (m.tab) {
+        case MenuTab::Home:      drawHomeTab(v, body); break;
+        case MenuTab::Settings:  drawSettingsTab(v, body); break;
+        case MenuTab::LoadWorld: drawLoadWorldTab(v, body); break;
+    }
+}
+
+void handleMainMenuInput(Viewer& v) {
+    MainMenuState& m = v.menuState;
+
+    // Typed numeric entry swallows other input while a field is focused (mirrors Edit Mode's
+    // ViewerInput.cpp typing loop / Viewer::commitEditField).
+    if (m.editingConfigField >= 0) {
+        int ch = GetCharPressed();
+        while (ch > 0) {
+            if ((ch >= '0' && ch <= '9') || ch == '.' || ch == '-') m.editBuffer += (char)ch;
+            ch = GetCharPressed();
+        }
+        if (IsKeyPressed(KEY_BACKSPACE) && !m.editBuffer.empty()) m.editBuffer.pop_back();
+        if (IsKeyPressed(KEY_ENTER)) v.commitMenuField();
+        if (IsKeyPressed(KEY_ESCAPE)) { m.editingConfigField = -1; m.editBuffer.clear(); }
+        return;
+    }
+
+    if (!IsMouseButtonPressed(MOUSE_BUTTON_LEFT)) return;
+    Vector2 mp = GetMousePosition();
+
+    for (size_t i = 0; i < std::size(kTabs); ++i) {
+        if (!CheckCollisionPointRec(mp, tabButtonRect((int)i))) continue;
+        m.tab = kTabs[i].tab;
+        m.editingConfigField = -1; m.editBuffer.clear();
+        if (m.tab == MenuTab::LoadWorld) m.saveFiles = listSaveFiles();
+        return;
+    }
+
+    Rectangle body = mainMenuBody(v);
+    if (m.tab == MenuTab::Home) {
+        HomeLayout L = computeHomeLayout(body);
+        if (CheckCollisionPointRec(mp, L.newBtn)) { v.startNewWorld(); return; }
+        if (m.worldEverGenerated && CheckCollisionPointRec(mp, L.backBtn)) { v.screen = AppScreen::World; return; }
+        if (CheckCollisionPointRec(mp, L.randBtn)) {
+            v.cfg.seed = (uint32_t)(GetTime() * 100000.0) | 1u;   // odd, non-zero: matches the R-key reseed convention
+            return;
+        }
+    } else if (m.tab == MenuTab::Settings) {
+        for (size_t i = 0; i < m.categoryRects.size(); ++i) {
+            if (!CheckCollisionPointRec(mp, m.categoryRects[i])) continue;
+            m.settingsCategory = (int)i;
+            return;
+        }
+    } else {   // LoadWorld
+        for (size_t i = 0; i < m.saveFileRects.size(); ++i) {
+            if (!CheckCollisionPointRec(mp, m.saveFileRects[i])) continue;
+            if (i < m.saveFiles.size()) v.loadGame(m.saveFiles[i].c_str());
+            return;
+        }
+    }
+
+    // Generic: click on any drawn config-field row (Home's seed row or a Settings-tab field)
+    // focuses it for typed entry, seeded with its current value.
+    for (size_t i = 0; i < m.fieldRects.size(); ++i) {
+        if (!CheckCollisionPointRec(mp, m.fieldRects[i])) continue;
+        int fieldIdx = m.fieldIndices[i];
+        const auto& d = configFieldTable()[fieldIdx];
+        m.editingConfigField = fieldIdx;
+        m.editBuffer = TextFormat(d.isInteger ? "%.0f" : "%.4g", d.get(v.cfg));
+        return;
+    }
+}
diff --git a/src/render/MainMenu.hpp b/src/render/MainMenu.hpp
new file mode 100644
index 0000000..e03b6cf
--- /dev/null
+++ b/src/render/MainMenu.hpp
@@ -0,0 +1,35 @@
+#pragma once
+#include "raylib.h"
+#include <string>
+#include <vector>
+
+struct Viewer;
+
+// The full-screen Main Menu: settings editor over every PlanetConfig field, a seed field,
+// "New World" / "Load World" / "Back to World" actions. Shown at app startup (before any world
+// has ever been generated) and reachable again later via a toolbar button, without disturbing a
+// running world unless the user explicitly picks New or Load.
+enum class AppScreen { MainMenu, World };
+enum class MenuTab { Home, Settings, LoadWorld };
+
+// Transient menu-UI state, owned by the Viewer, rebuilt each frame by drawMainMenu() (render
+// side) and consumed by handleMainMenuInput() -- mirrors EditPanelState's pattern in Panels.hpp.
+struct MainMenuState {
+    MenuTab tab = MenuTab::Home;
+    int  settingsCategory = 0;          // which left-rail category is open
+    int  editingConfigField = -1;       // index into configFieldTable(), -1 = nothing focused
+    std::string editBuffer;
+    bool worldEverGenerated = false;    // gates the "Back to World" button
+    Vector2 settingsScroll{0, 0};       // GuiScrollPanel offset for the field list
+
+    std::vector<Rectangle> categoryRects;       // left-rail hit rects
+    std::vector<Rectangle> fieldRects;          // parallel to fieldIndices: the open category's fields
+    std::vector<int>       fieldIndices;        // parallel to fieldRects: index into configFieldTable()
+    std::vector<std::string> saveFiles;         // Load World: paths, newest first
+    std::vector<Rectangle> saveFileRects;       // parallel to saveFiles
+};
+
+// No separate style setup: raygui's re-skin (setupToolbarStyle(), called once in Viewer::init())
+// is a global style context, already applied before either screen ever renders.
+void drawMainMenu(Viewer& v);       // draw the whole menu screen for this frame
+void handleMainMenuInput(Viewer& v); // mouse/keyboard handling while screen == AppScreen::MainMenu
diff --git a/src/render/Toolbar.cpp b/src/render/Toolbar.cpp
index 48cc952..761d344 100644
--- a/src/render/Toolbar.cpp
+++ b/src/render/Toolbar.cpp
@@ -195,12 +195,19 @@ void drawToolbar(Viewer& v) {
         if (GuiButton(takeRow(cx, cw, cy), "Reseed (R)")) v.reseed();
         {
             Rectangle r = takeRow(cx, cw, cy);
-            if (GuiButton(Rectangle{ r.x, r.y, halfW, r.height }, "Save (F5)")) v.saveGame(v.SAVE_PATH);
-            if (GuiButton(Rectangle{ r.x + halfW + GAP, r.y, halfW, r.height }, "Load (F9)")) v.loadGame(v.SAVE_PATH);
+            if (GuiButton(Rectangle{ r.x, r.y, halfW, r.height }, "Save (F5)")) v.saveGame(makeTimestampedSavePath().c_str());
+            if (GuiButton(Rectangle{ r.x + halfW + GAP, r.y, halfW, r.height }, "Load (F9)")) {
+                auto files = listSaveFiles();
+                if (!files.empty()) v.loadGame(files.front().c_str());
+                else v.setStatus("No saves found");
+            }
         }
         // Deferred to after drawToolbar() returns -- see Viewer::pendingMapExport/pendingAtlasExport.
         if (GuiButton(takeRow(cx, cw, cy), "Export Map (F11)")) v.pendingMapExport = true;
         if (GuiButton(takeRow(cx, cw, cy), "Export Atlas (Shift+F11)")) v.pendingAtlasExport = true;
+        // Doesn't touch planet/cfg/mode/overlays -- "Back to World" on the Home tab is a pure no-op
+        // return, so the running world is completely undisturbed.
+        if (GuiButton(takeRow(cx, cw, cy), "Main Menu")) { v.screen = AppScreen::MainMenu; v.menuState.tab = MenuTab::Home; }
         cy += SECTION_GAP;
 
         // --- Civilization & Ecology (all need a settled world) ------------------
diff --git a/src/render/Viewer.cpp b/src/render/Viewer.cpp
index 19eb605..472fc6a 100644
--- a/src/render/Viewer.cpp
+++ b/src/render/Viewer.cpp
@@ -5,6 +5,7 @@
 #include <algorithm>
 #include <cmath>
 #include <cstring>
+#include <ctime>
 #include <fstream>
 #include <set>
 #include <utility>
@@ -129,18 +130,15 @@ bool Viewer::init(int argc, char** argv) {
     if (cliSeed != 0) cfg.seed = cliSeed;                  // CLI --seed overrides config
     std::string cfgErr = validateConfig(cfg);
     if (!cfgErr.empty()) cfg = PlanetConfig{};             // revert to safe defaults
-    planet.generate(cfg);
+    // No planet.generate() here anymore -- the app opens on the Main Menu; a world is only
+    // built once the user picks "New World" (startNewWorld()) or "Load World" (loadGame()).
 
     cam.position = {0, 0, 6}; cam.target = {0, 0, 0}; cam.up = {0, 1, 0};
     cam.fovy = 45; cam.projection = CAMERA_PERSPECTIVE;
 
-    buildBorders(planet, borderR, borders, ridgeBorders);
-    buildDriftArrows(planet, driftR, driftArrows, plateLabels);
-    graticule = buildGraticule();
-    buildMap2D(planet, mapRect, map2D);
-    phase3PromptAt = planet.cfg.phase3AfterMy;
+    graticule = buildGraticule();   // planet-independent, safe to build before any world exists
 
-    refreshView();   // colour the freshly generated (flat) world
+    screen = AppScreen::MainMenu;
     return true;
 }
 
@@ -364,6 +362,22 @@ void Viewer::regenWorld() {                 // after generate(): geometry change
 
 void Viewer::regen() { planet.generate(cfg); regenWorld(); }
 
+void Viewer::startNewWorld() {               // Main Menu "New World": validate, generate, switch screens
+    std::string err = validateConfig(cfg);
+    if (!err.empty()) { cfg = PlanetConfig{}; setStatus("Bad settings -- using defaults"); }
+    regen();
+    menuState.worldEverGenerated = true;
+    menuState.editingConfigField = -1; menuState.editBuffer.clear();
+    screen = AppScreen::World;
+}
+
+void Viewer::commitMenuField() {             // parse menuState.editBuffer, apply to the focused config field
+    const auto& t = configFieldTable();
+    if (menuState.editingConfigField < 0 || menuState.editingConfigField >= (int)t.size()) return;
+    try { t[menuState.editingConfigField].set(cfg, std::stod(menuState.editBuffer)); } catch (...) {}
+    menuState.editingConfigField = -1; menuState.editBuffer.clear();
+}
+
 void Viewer::stepOnce() {                    // one tick + settle bookkeeping
     maxChange = planet.step(); ++stepCount;
     if (maxChange < settleThresh) { if (++settleRun >= settleNeed) settled = true; }
@@ -552,6 +566,26 @@ void Viewer::focusCell(int idx, const std::string& status) {
     if (!status.empty()) setStatus(status);
 }
 
+// F5 / the toolbar "Save" button always write a fresh timestamped file (mirrors the
+// map/atlas export naming scheme, ViewerRender.cpp); F9 / "Load" pick the newest one.
+std::string makeTimestampedSavePath() {
+    time_t now = time(nullptr);
+    struct tm tmv; localtime_r(&now, &tmv);
+    char name[64];
+    strftime(name, sizeof(name), "world_%Y-%m-%d_%H%M%S.save", &tmv);
+    return name;
+}
+
+std::vector<std::string> listSaveFiles() {
+    FilePathList files = LoadDirectoryFilesEx(".", ".save", false);
+    std::vector<std::string> out;
+    out.reserve(files.count);
+    for (unsigned i = 0; i < files.count; ++i) out.emplace_back(files.paths[i]);
+    UnloadDirectoryFiles(files);
+    std::sort(out.begin(), out.end(), std::greater<>());   // fixed timestamp format sorts chronologically
+    return out;
+}
+
 // F5: write seed + config + full planet state. F9: read it back and resume.
 void Viewer::saveGame(const char* path) {
     std::ofstream os(path, std::ios::binary);
@@ -614,6 +648,25 @@ void Viewer::saveGame(const char* path) {
         os.write((char*)&e.sourceId, 4);
         wS(e.title); wS(e.detail);
     }
+    // v25: the viewer's colour mode + overlay toggles, so a load restores what you were looking at,
+    // not just the simulation. Camera angle / map zoom-pan / selected cell stay session-local.
+    uint8_t modeByte = (uint8_t)mode;
+    os.write((char*)&modeByte, 1);
+    uint16_t overlayBits = 0;
+    overlayBits |= showBorders     ? 1u << 0  : 0;
+    overlayBits |= showDrift       ? 1u << 1  : 0;
+    overlayBits |= showRivers      ? 1u << 2  : 0;
+    overlayBits |= showGrat        ? 1u << 3  : 0;
+    overlayBits |= dayNightOn      ? 1u << 4  : 0;
+    overlayBits |= showTides       ? 1u << 5  : 0;
+    overlayBits |= showCurrents    ? 1u << 6  : 0;
+    overlayBits |= showClouds      ? 1u << 7  : 0;
+    overlayBits |= showVolcanoes   ? 1u << 8  : 0;
+    overlayBits |= showNames       ? 1u << 9  : 0;
+    overlayBits |= showSettlements ? 1u << 10 : 0;
+    overlayBits |= showDiplomacy   ? 1u << 11 : 0;
+    overlayBits |= showRealms      ? 1u << 12 : 0;
+    os.write((char*)&overlayBits, sizeof overlayBits);
     setStatus(os ? std::string("Saved ") + path : "Save failed");
 }
 
@@ -759,6 +812,13 @@ void Viewer::loadGame(const char* path) {
         }
         if (!eventsOk) { events.clear(); nextEventId = 1; skippedHistory = true; }
     }
+    // v25: colour mode + overlay toggles, written at the very end of saveGame() (after the events
+    // block), so read them from the same position here.
+    uint8_t modeByte = 0; uint16_t overlayBits = 0;
+    if (ver >= 25) {
+        is.read(reinterpret_cast<char*>(&modeByte), 1);
+        is.read(reinterpret_cast<char*>(&overlayBits), sizeof overlayBits);
+    }
     paused = true; selectedCell = -1; subgrids.clear();
     // Pre-v14 save already in Live World: it has no volcano block, so place a set now (v14+ saves
     // restore their own). A non-live save places them when the user first presses W.
@@ -767,7 +827,27 @@ void Viewer::loadGame(const char* path) {
     buildDriftArrows(planet, driftR, driftArrows, plateLabels);
     buildMap2D(planet, mapRect, map2D);
     if (planet.settlementsPlaced()) rebuildTerritory();   // territory/nations are derived -> recompute
+    // v25: restore the colour mode + overlay toggles. Pre-v25 saves leave them at whatever the
+    // Viewer already held (same "session state survives a load" behaviour saves have always had).
+    if (ver >= 25) {
+        if (modeByte <= (uint8_t)ColorMode::Seasonality) mode = (ColorMode)modeByte;
+        showBorders     = overlayBits & (1u << 0);
+        showDrift       = overlayBits & (1u << 1);
+        showRivers      = overlayBits & (1u << 2);
+        showGrat        = overlayBits & (1u << 3);
+        dayNightOn      = overlayBits & (1u << 4);
+        showTides       = overlayBits & (1u << 5);
+        showCurrents    = overlayBits & (1u << 6);
+        showClouds      = overlayBits & (1u << 7);
+        showVolcanoes   = overlayBits & (1u << 8);
+        showNames       = overlayBits & (1u << 9);
+        showSettlements = overlayBits & (1u << 10);
+        showDiplomacy   = overlayBits & (1u << 11);
+        showRealms      = overlayBits & (1u << 12);
+    }
     refreshView();
+    menuState.worldEverGenerated = true;   // a loaded world can always be returned to from the Main Menu
+    screen = AppScreen::World;
     setStatus(skippedHistory ? std::string("Loaded ") + path + " (history skipped)"
                              : std::string("Loaded ") + path);
 }
@@ -1104,9 +1184,11 @@ Rectangle Viewer::mapViewRect() const {
 void Viewer::run() {
     while (!WindowShouldClose()) {
         handleInput();
-        stepSim();
-        // A regenerate this frame may have shrunk the planet; keep indices valid.
-        if (hovered >= (int)planet.cells.size()) hovered = -1;
+        if (screen == AppScreen::World) {
+            stepSim();
+            // A regenerate this frame may have shrunk the planet; keep indices valid.
+            if (hovered >= (int)planet.cells.size()) hovered = -1;
+        }
         renderFrame();
     }
     UnloadRenderTexture(rt3d);
diff --git a/src/render/Viewer.hpp b/src/render/Viewer.hpp
index 2abcab8..0db5aba 100644
--- a/src/render/Viewer.hpp
+++ b/src/render/Viewer.hpp
@@ -5,6 +5,7 @@
 #include "Overlays.hpp"   // PlateLabel
 #include "Map2D.hpp"      // Map2D
 #include "Panels.hpp"     // EditField, EditPanelState
+#include "MainMenu.hpp"   // AppScreen, MenuTab, MainMenuState
 #include <vector>
 #include <string>
 #include <memory>
@@ -16,13 +17,16 @@
 // ViewerInput.cpp (input/picking/keys) and ViewerRender.cpp (drawing).
 struct Viewer {
     // ---- Files / save format ------------------------------------------------
-    static constexpr uint32_t SAVE_VERSION = 24;  // v24: edit mode (per-cell lock bits + locked-value maps); v23: civ cultural evolution; v22: civ diplomacy; v21: civ conflict/wars; v20: civ settlements; v19: ecoregions; v18: geography reshuffle salt; v17: +geography/atlas; v16: +event log; v15: stateful volcanoes; v14: old volcanoes; v13: +liveRate; v12: +step-back history; v11: +weather systems; v10: +weather fields; v9: +moons; v8: +Live World clock; v7: +biota; v6: self-describing config; v4: +biome; v3: +phase3
+    static constexpr uint32_t SAVE_VERSION = 25;  // v25: +viewer colour-mode/overlay state (Viewer::saveGame/loadGame only, not Planet); v24: edit mode (per-cell lock bits + locked-value maps); v23: civ cultural evolution; v22: civ diplomacy; v21: civ conflict/wars; v20: civ settlements; v19: ecoregions; v18: geography reshuffle salt; v17: +geography/atlas; v16: +event log; v15: stateful volcanoes; v14: old volcanoes; v13: +liveRate; v12: +step-back history; v11: +weather systems; v10: +weather fields; v9: +moons; v8: +Live World clock; v7: +biota; v6: self-describing config; v4: +biome; v3: +phase3
     static constexpr int wxSaveMax = 40;          // most recent step-back frames persisted in a save
     static constexpr int EVENT_LOG_MAX = 200;
     const char* CONFIG_PATH = "planet.cfg";
-    const char* SAVE_PATH   = "planet.save";
     std::string configPath  = "planet.cfg";       // initial config (--config overrides)
 
+    // ---- App screen (Main Menu vs. a live world) ----------------------------
+    AppScreen     screen = AppScreen::MainMenu;   // init() no longer auto-generates a world
+    MainMenuState menuState;
+
     // ---- Window / layout (set in init) --------------------------------------
     int screenW = 1920, screenH = 1080;
     int leftW = 0, rightX = 0, rightW = 0, rightH = 0;
@@ -211,6 +215,8 @@ struct Viewer {
     const std::vector<Color>& displayColors() const { return liveWorld ? shadedColors : vcolors; }
     void regenWorld();                   // after generate(): geometry changed
     void regen();                        // generate(cfg) + regenWorld()
+    void startNewWorld();                // validate cfg, regen(), switch screen to World (Main Menu "New World")
+    void commitMenuField();              // parse menuState.editBuffer and apply it to the focused config field
     void stepOnce();                     // one tick + settle bookkeeping
     void pauseAction();
     void setStatus(const std::string& m);
@@ -278,3 +284,8 @@ struct Viewer {
     void renderHUD();
     void renderPrompt();
 };
+
+// Free helpers for the timestamped save scheme (no Viewer state needed):
+// F5 / the toolbar "Save" button always write a fresh file; F9 / "Load" pick the newest.
+std::string makeTimestampedSavePath();          // "world_YYYY-MM-DD_HHMMSS.save"
+std::vector<std::string> listSaveFiles();       // *.save in the working dir, newest first
diff --git a/src/render/ViewerInput.cpp b/src/render/ViewerInput.cpp
index 03eca0f..0087a30 100644
--- a/src/render/ViewerInput.cpp
+++ b/src/render/ViewerInput.cpp
@@ -2,6 +2,7 @@
 #include "Picking.hpp"
 #include "Map2D.hpp"        // wrapPi
 #include "Projection.hpp"   // EqualEarth, lonLatToDir
+#include "MainMenu.hpp"     // handleMainMenuInput
 #include <algorithm>
 #include <cmath>
 
@@ -10,6 +11,7 @@
 // members (mp/onPause/hovered/hasHoverSub/hoverSub/hoveredSubIdx) for the render
 // pass, and may step / regenerate / save / load the world.
 void Viewer::handleInput() {
+    if (screen == AppScreen::MainMenu) { handleMainMenuInput(*this); return; }
     mp = GetMousePosition();
     bool in3D     = (mp.x < view3DW && mp.y < view3DH);   // top-left quadrant
     bool inMap    = CheckCollisionPointRec(mp, mapRect);
@@ -241,8 +243,12 @@ void Viewer::handleInput() {
             regen();
             setStatus(cerr.empty() ? std::string("Reloaded ") + configPath : "Bad config — using defaults");
         } else setStatus(std::string("No ") + configPath); }
-    if (IsKeyPressed(KEY_F5)) saveGame(SAVE_PATH);
-    if (IsKeyPressed(KEY_F9)) loadGame(SAVE_PATH);
+    if (IsKeyPressed(KEY_F5)) saveGame(makeTimestampedSavePath().c_str());
+    if (IsKeyPressed(KEY_F9)) {
+        auto files = listSaveFiles();
+        if (!files.empty()) loadGame(files.front().c_str());
+        else setStatus("No saves found");
+    }
     if (IsKeyPressed(KEY_F12)) { TakeScreenshot("screenshot.png"); setStatus("Screenshot saved to screenshot.png"); }
     if (IsKeyPressed(KEY_F11)) {
         if (IsKeyDown(KEY_LEFT_SHIFT) || IsKeyDown(KEY_RIGHT_SHIFT)) exportAtlasImage();
diff --git a/src/render/ViewerRender.cpp b/src/render/ViewerRender.cpp
index 83da4fc..1560295 100644
--- a/src/render/ViewerRender.cpp
+++ b/src/render/ViewerRender.cpp
@@ -3,6 +3,7 @@
 #include "Map2D.hpp"
 #include "Panels.hpp"
 #include "Toolbar.hpp"      // drawToolbar
+#include "MainMenu.hpp"     // drawMainMenu
 #include "Picking.hpp"      // rotateZ (axial-tilt transform for labels)
 #include "rlgl.h"
 #include "Projection.hpp"   // dirToLonLat (plate labels)
@@ -1169,6 +1170,15 @@ void Viewer::renderPrompt() {
 // One full frame: globe texture, then composite + 3D labels + map + panels +
 // HUD + prompt onto the screen.
 void Viewer::renderFrame() {
+    // A complete early return, not a dim overlay over a running world (unlike the Phase-3 modal
+    // below) -- the Main Menu must render before any world/planet.cells even exists.
+    if (screen == AppScreen::MainMenu) {
+        BeginDrawing();
+        ClearBackground(Color{8, 10, 16, 255});
+        drawMainMenu(*this);
+        EndDrawing();
+        return;
+    }
     renderGlobe3D();
 
     BeginDrawing();
diff --git a/src/sim/Planet.hpp b/src/sim/Planet.hpp
index 65638ba..c18a97c 100644
--- a/src/sim/Planet.hpp
+++ b/src/sim/Planet.hpp
@@ -483,3 +483,19 @@ private:
 bool loadConfig(const std::string& path, PlanetConfig& cfg);
 bool saveConfig(const std::string& path, const PlanetConfig& cfg);
 std::string validateConfig(const PlanetConfig& cfg);
+
+// One entry per CONFIG_FIELDS token (see PlanetIO.cpp), in PlanetConfig declaration order, built
+// once at first use. Lets a single generic UI loop (the Main Menu settings editor) drive all ~240
+// config fields instead of 240 hand-written widgets -- the same "add a field, everything else keeps
+// working" property writeConfigFields()/parseConfigStream() already have for planet.cfg/saves.
+struct ConfigFieldDesc {
+    const char* name;                       // matches the CONFIG_FIELDS token, e.g. "seaLevel"
+    const char* category;                   // section label, e.g. "Tectonic tuning"
+    bool        isInteger;                  // true for I()/U() fields (whole-number display/parse)
+    double (*get)(const PlanetConfig&);
+    void   (*set)(PlanetConfig&, double);
+};
+const std::vector<ConfigFieldDesc>& configFieldTable();
+// The distinct ConfigFieldDesc::category labels, in display order (25 groups) -- the Main Menu
+// settings editor's left-rail category list.
+const std::vector<const char*>& configCategoryList();
diff --git a/src/sim/PlanetIO.cpp b/src/sim/PlanetIO.cpp
index 5b018d7..70e4939 100644
--- a/src/sim/PlanetIO.cpp
+++ b/src/sim/PlanetIO.cpp
@@ -10,6 +10,7 @@
 #include <limits>
 #include <set>
 #include <utility>
+#include <cstring>
 
 // --- Config file (text) + save/load (binary) --------------------------------
 
@@ -115,6 +116,206 @@ static void parseConfigStream(std::istream& is, PlanetConfig& cfg) {
     }
 }
 
+// Category assignment: an explicit name->category table, built by hand from PlanetConfig's actual
+// struct declaration order in PlanetTypes.hpp (25 "// ---" / plain-comment sections; "Plate
+// lifecycle" absorbs the trailing "Spreading & volcanic (legacy)" block and "Cultural evolution"
+// absorbs "Dawn seeding", matching how CLAUDE.md documents those as one knob group each). File
+// scope (not local to buildConfigFieldTable()) so configCategoryList() can share it below.
+//
+// This CANNOT be a "first field of each section" boundary scan over configFieldTable()'s own
+// iteration order -- CONFIG_FIELDS expands to all D() fields, then all I() fields, then U(seed),
+// so within a section whose fields mix types (nearly all of them) the section's members are
+// scattered across two separate runs of the table, not contiguous. A name->category lookup sidesteps
+// that entirely. This array IS written in one contiguous block per category, though (unlike
+// configFieldTable()), so it doubles as the source of truth for configCategoryList()'s display order.
+static const struct { const char* name; const char* category; } kConfigFieldCategory[] = {
+    {"radius", "Core"}, {"subdivisions", "Core"}, {"seaLevel", "Core"}, {"plateCount", "Core"},
+    {"seed", "Core"}, {"axialTilt", "Core"},
+
+    {"continentBase", "Tectonic tuning"}, {"oceanBase", "Tectonic tuning"},
+    {"upliftGain", "Tectonic tuning"}, {"beltWidth", "Tectonic tuning"}, {"relax", "Tectonic tuning"},
+
+    {"collisionFactor", "Orogeny"}, {"arcFactor", "Orogeny"}, {"isostaticPersist", "Orogeny"},
+    {"rootScale", "Orogeny"},
+
+    {"peakSoftCapStart", "Soft peak cap"}, {"peakSoftCapEnd", "Soft peak cap"},
+    {"peakFailDrop", "Soft peak cap"},
+
+    {"maxDriftSpeed", "Plate drift"}, {"ridgeDepth", "Plate drift"},
+
+    {"seafloorSubsidence", "Seafloor aging"}, {"seafloorSeedAge", "Seafloor aging"},
+
+    {"splitCheckEvery", "Plate lifecycle"}, {"splitFraction", "Plate lifecycle"},
+    {"splitProbBase", "Plate lifecycle"}, {"splitProbSlope", "Plate lifecycle"},
+    {"stalemateEps", "Plate lifecycle"}, {"stalemateWindows", "Plate lifecycle"},
+    {"stalemateBoost", "Plate lifecycle"}, {"miniPlateCells", "Plate lifecycle"},
+    {"fuseMinPlates", "Plate lifecycle"}, {"babyMinCells", "Plate lifecycle"},
+    {"babyPromoteFrac", "Plate lifecycle"}, {"volcanicLandFrac", "Plate lifecycle"},
+    {"volcanicElev", "Plate lifecycle"}, {"landBand", "Plate lifecycle"},
+
+    {"erosionLandRate", "Erosion & sea level"}, {"erosionSeaRate", "Erosion & sea level"},
+    {"landFractionTarget", "Erosion & sea level"}, {"seaLevelStep", "Erosion & sea level"},
+    {"seaLevelTol", "Erosion & sea level"}, {"seaLevelEvery", "Erosion & sea level"},
+
+    {"phase3AfterMy", "Hydrology"}, {"phase3DtScale", "Hydrology"}, {"rainfall", "Hydrology"},
+    {"riverThreshold", "Hydrology"}, {"riverIncision", "Hydrology"},
+    {"riverDischargeExp", "Hydrology"}, {"riverSlopeExp", "Hydrology"},
+    {"riverTransport", "Hydrology"}, {"depFrac", "Hydrology"},
+
+    {"biomeEquatorTemp", "Biome classification"}, {"biomePoleDrop", "Biome classification"},
+    {"biomeLatExp", "Biome classification"}, {"biomeElevLapse", "Biome classification"},
+    {"biomeIceTemp", "Biome classification"}, {"biomeTundraTemp", "Biome classification"},
+    {"biomeTaigaTemp", "Biome classification"}, {"biomeSavannaTemp", "Biome classification"},
+    {"biomeMountainElev", "Biome classification"}, {"biomeHillsElev", "Biome classification"},
+    {"biomeBeachBand", "Biome classification"}, {"biomeLowlandElev", "Biome classification"},
+    {"biomeWetlandMoist", "Biome classification"}, {"biomeDesertMoist", "Biome classification"},
+    {"biomeGrassMoist", "Biome classification"}, {"biomeTaigaMoist", "Biome classification"},
+    {"biomeLakeMinDepth", "Biome classification"}, {"biomeSeasonWeight", "Biome classification"},
+
+    {"climateOceanMoisture", "Climate"}, {"climateRainEfficiency", "Climate"},
+    {"climateOrographic", "Climate"}, {"climateOroRefHeight", "Climate"},
+    {"climateContinentality", "Climate"}, {"climateWindPasses", "Climate"},
+    {"climateMoistureSmooth", "Climate"}, {"climateCurrentFactor", "Climate"},
+
+    {"seasonAmpMax", "Seasons"}, {"seasonLatExp", "Seasons"}, {"seasonOceanFactor", "Seasons"},
+    {"seasonContinentRings", "Seasons"},
+
+    {"bioVegTempMin", "Biota"}, {"bioVegTempOpt", "Biota"}, {"bioVegMoistRef", "Biota"},
+    {"bioFaunaProductivity", "Biota"}, {"bioCarnPreyMin", "Biota"}, {"bioCarnScale", "Biota"},
+    {"bioFungaMoistRef", "Biota"}, {"bioFungaFloraWeight", "Biota"}, {"bioFungaTempMin", "Biota"},
+    {"bioRegionBonus", "Biota"}, {"bioMarineBase", "Biota"}, {"bioMarineShelfDepth", "Biota"},
+    {"bioMarineCoastRings", "Biota"}, {"bioFloraSlots", "Biota"}, {"bioFaunaSlots", "Biota"},
+    {"bioFungaSlots", "Biota"}, {"bioFloraPoints", "Biota"}, {"bioFaunaPoints", "Biota"},
+    {"bioFungaPoints", "Biota"},
+
+    {"dayLengthHours", "Live World"}, {"yearLengthDays", "Live World"}, {"snowTemp", "Live World"},
+    {"seaIceTemp", "Live World"}, {"tideAmplitude", "Live World"}, {"tideSunFactor", "Live World"},
+
+    {"weatherEvapRate", "Weather"}, {"weatherWindKmh", "Weather"}, {"weatherSatBase", "Weather"},
+    {"weatherSatTempCoef", "Weather"}, {"weatherCondense", "Weather"},
+    {"weatherOrographic", "Weather"}, {"weatherRainThresh", "Weather"},
+    {"weatherRainRate", "Weather"}, {"weatherCloudDissip", "Weather"},
+
+    {"weatherSystemMax", "Weather systems"}, {"weatherSpawnRate", "Weather systems"},
+    {"weatherSystemSpeed", "Weather systems"}, {"weatherTropicalSST", "Weather systems"},
+    {"weatherSystemRadius", "Weather systems"}, {"weatherSystemCloud", "Weather systems"},
+    {"weatherSystemRain", "Weather systems"}, {"weatherHurricaneStr", "Weather systems"},
+
+    {"volcanoProbRidge", "Volcanoes"}, {"volcanoProbBorder", "Volcanoes"},
+    {"volcanoProbInterior", "Volcanoes"}, {"volcanoMaxCount", "Volcanoes"},
+    {"volcanoBuildRate", "Volcanoes"}, {"volcanoFreeHeight", "Volcanoes"},
+    {"volcanoInitialBuildMax", "Volcanoes"}, {"volcanoMaxHeight", "Volcanoes"},
+    {"volcanoDormancyRate", "Volcanoes"}, {"volcanoDormantMinYears", "Volcanoes"},
+    {"volcanoDormantMaxYears", "Volcanoes"}, {"volcanoExplodeDropFrac", "Volcanoes"},
+    {"volcanoActivityDecay", "Volcanoes"}, {"volcanoDeadActivity", "Volcanoes"},
+    {"volcanoBlastRadius", "Volcanoes"}, {"volcanoBlastCloud", "Volcanoes"},
+    {"volcanoAshMinYears", "Volcanoes"}, {"volcanoAshMaxYears", "Volcanoes"},
+    {"volcanoAshPuffCellsPerWeek", "Volcanoes"}, {"volcanoAshCloud", "Volcanoes"},
+    {"volcanoAshCooling", "Volcanoes"},
+
+    {"geoContinentMinCells", "Geography"}, {"geoSeaMaxCells", "Geography"},
+    {"geoOceanSepRadians", "Geography"}, {"geoOceanDeep", "Geography"},
+    {"geoMountainElev", "Geography"}, {"geoRangeMinCells", "Geography"},
+    {"geoRiverMinDischarge", "Geography"}, {"geoMaxRivers", "Geography"}, {"geoMaxPeaks", "Geography"},
+
+    {"civMaxSettlements", "Civilization & settlements"}, {"civMinSpacingRadians", "Civilization & settlements"},
+    {"civClusterExp", "Civilization & settlements"}, {"civMinHabitability", "Civilization & settlements"},
+    {"civSeedPopulation", "Civilization & settlements"}, {"civGrowthRate", "Civilization & settlements"},
+    {"civMaxPopulation", "Civilization & settlements"}, {"civTownPop", "Civilization & settlements"},
+    {"civCityPop", "Civilization & settlements"}, {"civAbandonPop", "Civilization & settlements"},
+    {"civHabWaterWeight", "Civilization & settlements"}, {"civHabFoodWeight", "Civilization & settlements"},
+    {"civHabTempOpt", "Civilization & settlements"}, {"civHabElevPenalty", "Civilization & settlements"},
+    {"civSiteVariety", "Civilization & settlements"}, {"civGrowthMin", "Civilization & settlements"},
+    {"civHarvestVar", "Civilization & settlements"}, {"civDroughtStrength", "Civilization & settlements"},
+    {"civDroughtPeriod", "Civilization & settlements"}, {"civDroughtThresh", "Civilization & settlements"},
+    {"civDroughtArid", "Civilization & settlements"}, {"civColdYearStrength", "Civilization & settlements"},
+    {"civFloodBonus", "Civilization & settlements"}, {"civFamineRate", "Civilization & settlements"},
+    {"civStormDeathRate", "Civilization & settlements"}, {"civHurricaneDeathMult", "Civilization & settlements"},
+    {"civMetropolisPop", "Civilization & settlements"}, {"civCrowdingLoss", "Civilization & settlements"},
+    {"civCondBoomCap", "Civilization & settlements"}, {"civPlagueRate", "Civilization & settlements"},
+    {"civPlagueDeathMin", "Civilization & settlements"}, {"civPlagueDeathMax", "Civilization & settlements"},
+    {"civPlagueTradeWeight", "Civilization & settlements"},
+
+    {"civTerritoryBase", "Territory & nations"}, {"civTerritoryScale", "Territory & nations"},
+    {"civTerritoryMax", "Territory & nations"}, {"civVassalRange", "Territory & nations"},
+    {"civEmpireMinMembers", "Territory & nations"}, {"civEmpirePop", "Territory & nations"},
+
+    {"warMaxConcurrent", "Conflict & war"}, {"warDeclareRate", "Conflict & war"},
+    {"warAmbition", "Conflict & war"}, {"warIdeology", "Conflict & war"}, {"warBorder", "Conflict & war"},
+    {"warWarlikeMult", "Conflict & war"}, {"warCasualtyRate", "Conflict & war"},
+    {"warConquerScore", "Conflict & war"}, {"warSackChance", "Conflict & war"},
+    {"warExhaustion", "Conflict & war"}, {"warRevoltRate", "Conflict & war"},
+    {"warMinRealmPop", "Conflict & war"},
+
+    {"diploDriftRate", "Diplomacy"}, {"diploAffinity", "Diplomacy"}, {"diploWarPenalty", "Diplomacy"},
+    {"diploTruceYears", "Diplomacy"}, {"diploWarGrudge", "Diplomacy"},
+    {"diploAllyThreshold", "Diplomacy"}, {"diploNonAggThreshold", "Diplomacy"},
+    {"diploRivalThreshold", "Diplomacy"},
+
+    {"tradeLandRange", "Trade & economy"}, {"tradeSeaRange", "Trade & economy"},
+    {"tradeRiverBonus", "Trade & economy"}, {"tradeMinVolume", "Trade & economy"},
+    {"tradeProsperityWeight", "Trade & economy"}, {"tradeWarBlock", "Trade & economy"},
+    {"tradeAllyBonus", "Trade & economy"}, {"tradeDiploBonus", "Trade & economy"},
+    {"tradeTemptWar", "Trade & economy"},
+
+    {"civColonizeRate", "Colonization"}, {"civColonyMinPop", "Colonization"},
+    {"civColonyMinHab", "Colonization"}, {"civColonyReach", "Colonization"},
+    {"civColonySeaReach", "Colonization"}, {"civColonySpacing", "Colonization"},
+    {"civMaxColonies", "Colonization"}, {"civColonySupply", "Colonization"},
+
+    {"cultAssimRate", "Cultural evolution"}, {"cultConvertRate", "Cultural evolution"},
+    {"cultConvertDominance", "Cultural evolution"}, {"cultSpreadRange", "Cultural evolution"},
+    {"cultPrestigeWeight", "Cultural evolution"}, {"cultSchismMinMembers", "Cultural evolution"},
+    {"cultSchismRange", "Cultural evolution"}, {"cultSchismMinCluster", "Cultural evolution"},
+    {"cultSchismRate", "Cultural evolution"}, {"cultDawnSettlementsPerCulture", "Cultural evolution"},
+    {"cultDawnMaxPerContinent", "Cultural evolution"},
+};
+
+// A runtime-iterable field table built from the same CONFIG_FIELDS macro, once, so a single
+// generic UI loop (the Main Menu settings editor) can drive all ~240 fields instead of 240
+// hand-written widgets -- the same "add a field, everything else keeps working" property
+// writeConfigFields()/parseConfigStream() already give planet.cfg/saves above.
+static std::vector<ConfigFieldDesc> buildConfigFieldTable() {
+    std::vector<ConfigFieldDesc> t;
+    #define FIELD_D(name) t.push_back({#name, "", false, \
+        [](const PlanetConfig& c){ return c.name; }, \
+        [](PlanetConfig& c, double v){ c.name = v; }});
+    #define FIELD_I(name) t.push_back({#name, "", true, \
+        [](const PlanetConfig& c){ return (double)c.name; }, \
+        [](PlanetConfig& c, double v){ c.name = (int)std::lround(v); }});
+    #define FIELD_U(name) t.push_back({#name, "", true, \
+        [](const PlanetConfig& c){ return (double)c.name; }, \
+        [](PlanetConfig& c, double v){ c.name = (uint32_t)std::llround(v); }});
+    CONFIG_FIELDS(FIELD_D, FIELD_I, FIELD_U)
+    #undef FIELD_D
+    #undef FIELD_I
+    #undef FIELD_U
+
+    for (auto& d : t) {
+        d.category = "Uncategorized";   // must never survive: every name appears in kConfigFieldCategory
+        for (const auto& c : kConfigFieldCategory)
+            if (std::strcmp(d.name, c.name) == 0) { d.category = c.category; break; }
+    }
+    return t;
+}
+const std::vector<ConfigFieldDesc>& configFieldTable() {
+    static const std::vector<ConfigFieldDesc> t = buildConfigFieldTable();
+    return t;
+}
+
+// The distinct category labels in kConfigFieldCategory's own written order (which, unlike
+// configFieldTable(), IS one contiguous block per category) -- the Main Menu settings editor's
+// left-rail category order.
+const std::vector<const char*>& configCategoryList() {
+    static const std::vector<const char*> cats = [] {
+        std::vector<const char*> out;
+        for (const auto& c : kConfigFieldCategory)
+            if (out.empty() || std::strcmp(out.back(), c.category) != 0) out.push_back(c.category);
+        return out;
+    }();
+    return cats;
+}
+
 bool saveConfig(const std::string& path, const PlanetConfig& cfg) {
     std::ofstream os(path);
     if (!os) return false;
diff --git a/test_viewerstate.cpp b/test_viewerstate.cpp
new file mode 100644
index 0000000..8215350
--- /dev/null
+++ b/test_viewerstate.cpp
@@ -0,0 +1,80 @@
+// Headless checks for the v25 viewer colour-mode/overlay save block (no window needed).
+// Modeled on test_events.cpp.
+
+#include "Viewer.hpp"
+#include <cstdio>
+#include <fstream>
+#include <string>
+
+static int failures = 0;
+static void check(bool cond, const char* what) {
+    std::printf("  [%s] %s\n", cond ? "PASS" : "FAIL", what);
+    if (!cond) ++failures;
+}
+
+int main() {
+    std::printf("Viewer state: v25 colour-mode/overlay save/load round-trip\n");
+    Viewer v;
+    PlanetConfig cfg; cfg.subdivisions = 2; cfg.seed = 777;
+    v.cfg = cfg;
+    v.planet.generate(cfg);
+
+    // Set every overlay to a value that differs from the Viewer's own defaults, plus a non-default
+    // colour mode, so a bug that left fields untouched (rather than round-tripped) would show up.
+    v.mode = ColorMode::Biome;
+    v.showBorders = false;      // default true
+    v.showDrift = false;        // default true
+    v.showRivers = false;       // default true
+    v.showGrat = true;          // default false
+    v.dayNightOn = false;       // default true
+    v.showTides = true;         // default false
+    v.showCurrents = true;      // default false
+    v.showClouds = false;       // default true
+    v.showVolcanoes = false;    // default true
+    v.showNames = true;         // default false
+    v.showSettlements = false;  // default true
+    v.showDiplomacy = false;    // default true
+    v.showRealms = true;        // default false
+
+    const char* path = "/tmp/fanworgen_viewerstate_test.save";
+    v.saveGame(path);
+
+    Viewer r;
+    r.loadGame(path);
+    check(r.mode == ColorMode::Biome, "v25 save/load restores the colour mode");
+    check(r.showBorders == false, "v25 restores showBorders");
+    check(r.showDrift == false, "v25 restores showDrift");
+    check(r.showRivers == false, "v25 restores showRivers");
+    check(r.showGrat == true, "v25 restores showGrat");
+    check(r.dayNightOn == false, "v25 restores dayNightOn");
+    check(r.showTides == true, "v25 restores showTides");
+    check(r.showCurrents == true, "v25 restores showCurrents");
+    check(r.showClouds == false, "v25 restores showClouds");
+    check(r.showVolcanoes == false, "v25 restores showVolcanoes");
+    check(r.showNames == true, "v25 restores showNames");
+    check(r.showSettlements == false, "v25 restores showSettlements");
+    check(r.showDiplomacy == false, "v25 restores showDiplomacy");
+    check(r.showRealms == true, "v25 restores showRealms");
+    check(r.screen == AppScreen::World, "loadGame() switches the app screen to World");
+    check(r.menuState.worldEverGenerated, "loadGame() marks a world as having been generated");
+
+    // Negative test: patch the save's version back to 24 (pre-v25, no mode/overlay block) and load
+    // into a fresh Viewer whose defaults differ from the saved-but-truncated values above -- must
+    // not crash, and must leave mode/overlays exactly at the fresh Viewer's own pre-load values.
+    {
+        std::fstream fs(path, std::ios::in | std::ios::out | std::ios::binary);
+        uint32_t oldVer = 24;
+        fs.seekp(4);
+        fs.write((char*)&oldVer, 4);
+    }
+    Viewer old;
+    ColorMode preLoadMode = old.mode;
+    bool preLoadBorders = old.showBorders, preLoadRealms = old.showRealms;
+    old.loadGame(path);
+    check(old.mode == preLoadMode, "pre-v25 saves leave the colour mode at its pre-load value");
+    check(old.showBorders == preLoadBorders, "pre-v25 saves leave showBorders at its pre-load value");
+    check(old.showRealms == preLoadRealms, "pre-v25 saves leave showRealms at its pre-load value");
+
+    std::printf(failures ? "\nFAILURES: %d\n" : "\nALL VIEWER STATE CHECKS PASSED\n", failures);
+    return failures ? 1 : 0;
+}