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/hud.cpp

@@ -53,14 +53,21 @@ thread_local bool g_eventShifted = false;
 // Canvas: a 16:9 screen (kCanvasWidth), centred.
 enum class Anchor { Center, Right, Screen, Canvas };
 
+// Offset of the main menu's 1024 frame while it is drawn centred (set each
+// menu frame by RenderMenuFrame; 0 while a 16:9 screen draws itself).
+std::atomic<int> g_menuFrameDx = 0;
+
 int OffsetFor(Anchor anchor) {
     static const bool enabled = re::ConfigInt("hud", "Enabled", 1) != 0;
     const int width = mods::widescreen::RenderWidth();
     if (anchor == Anchor::Canvas) {
         return width > kCanvasWidth ? (width - kCanvasWidth) / 2 : 0;
     }
-    if (!enabled || width <= kLayoutWidth || mods::widescreen::IsClassicFrame() ||
-        mods::widescreen::MenuIsShowing() || anchor == Anchor::Screen) {
+    // The main menu's frame shifts the windows in it; over a 16:9 screen
+    // (no frame) dialogs are centred like in a game (user, 2026-10-21).
+    const bool inMenuFrame = mods::widescreen::MenuIsShowing() && g_menuFrameDx != 0;
+    if (!enabled || width <= kLayoutWidth || mods::widescreen::IsClassicFrame() || inMenuFrame ||
+        anchor == Anchor::Screen) {
         return 0;
     }
     return anchor == Anchor::Center ? (width - kLayoutWidth) / 2 : width - kLayoutWidth;
@@ -868,10 +875,6 @@ int HandleCentred(void* event, int (*body)(void* context), void* context) {
     return body(context);
 }
 
-// Offset of the main menu's 1024 frame while it is drawn centred (set each
-// menu frame by RenderMenuFrame; 0 while a 16:9 screen draws itself).
-std::atomic<int> g_menuFrameDx = 0;
-
 void SetHiddenTextures(void* const* surfaces, int count) {
     for (int i = 0; i < kHiddenTextureMax; ++i) {
         g_hiddenTextures[i].store(i < count ? surfaces[i] : nullptr, std::memory_order_relaxed);