Shared chest, menu after a game, dialogs over 16:9 screens

- Shared chest: emptied and not saved from the moment a save game is read
  until it reloads; a hero saved inside a load wrote the new world's
  objects (stale ids) as the chest's items.
- Main menu after a game: shown 16:9 screens are tracked on the menu thread
  only and expire after 2 frames; a destroyed in-game window kept the menu
  unframed (old pictures, frame at the left).
- Dialogs over a 16:9 screen in the main menu are centred, their dimming
  covers the screen.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/mods/screen.cpp

@@ -4,8 +4,7 @@
 
 #include <algorithm>
 #include <atomic>
-#include <mutex>
-#include <set>
+#include <map>
 #include <vector>
 
 #include <windows.h>
@@ -46,15 +45,22 @@ constexpr int kHeadingHeight = 30;
 // frames).
 std::atomic<unsigned> g_frame{1};
 
-// The windows whose screen is shown: the main menu's frame is not drawn
-// centred while any is (the screen draws itself across the width).
-std::mutex g_shownLock;
-std::set<const void*> g_shown;
+// The windows whose screen is shown on the menu thread, with the menu frame
+// they were last drawn in: the main menu's frame is not drawn centred while
+// any is (the screen draws itself across the width). Entries not drawn for
+// kShownFrames frames are dropped: the UI is rebuilt when a game starts or
+// ends, and a window destroyed while shown kept the menu from being framed
+// (user, 2026-10-21: "the old main menu is rendering again"). Menu thread only.
+constexpr unsigned kShownFrames = 2;
+unsigned g_menuFrame = 0;
+std::map<const void*, unsigned> g_shown;
 
 void NoteShown(const void* window, bool shown) {
-    std::lock_guard guard(g_shownLock);
+    if (!widescreen::IsMenuThread()) {
+        return;
+    }
     if (shown) {
-        g_shown.insert(window);
+        g_shown[window] = g_menuFrame;
     } else {
         g_shown.erase(window);
     }
@@ -91,11 +97,6 @@ void HidePictures() {
     hud::SetHiddenTextures(surfaces.data(), static_cast<int>(surfaces.size()));
 }
 
-bool ShownAny() {
-    std::lock_guard guard(g_shownLock);
-    return !g_shown.empty();
-}
-
 // The game's logo top left (user, 2026-10-21: "the bright blueish one"),
 // kLogoMargin from the edges: FX_SACREDLOGO.TGA, the emblem (horns around a
 // ring of runes) glowing blue on black, no alpha, added onto the background as
@@ -134,8 +135,10 @@ void DrawRockWall(void* device, float width, float height) {
 
 } // namespace
 
-bool AnyShown() {
-    return ShownAny();
+bool BeginMenuFrame() {
+    ++g_menuFrame;
+    std::erase_if(g_shown, [](const auto& entry) { return g_menuFrame - entry.second > kShownFrames; });
+    return !g_shown.empty();
 }
 
 void DrawBackground(void* device) {
@@ -261,7 +264,7 @@ uint32_t __fastcall UiManager_Render(void* self, void* edx, uint32_t argument) {
     }
     auto* driver = *reinterpret_cast<DxDriver7**>(addr::g_dxDriver);
     void* device = driver ? driver->device : nullptr;
-    const bool framed = !screen::AnyShown();
+    const bool framed = !screen::BeginMenuFrame();
     if (framed && device) {
         screen::DrawBackground(device);
     }