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>

docs/shared-chest.md

@@ -70,3 +70,18 @@ was empty after loading another character (in-game saves don't call saveHero).
 - loadHero's extra fix-up for some item classes (`0x5beaa0` / `0x5d3a20`,
   item classes 5, 6, 8, 0xd, 0x11..0x19, 0x1b, 0x1d) is not repeated.
 - Multiplayer: not supported.
+
+## Items replaced by a world's (fixed 2026-10-21)
+
+Reported: the shared chest's items were gone, other items (potions, gear) in
+their cells. The grid holds object ids. A save game loaded into the running
+engine (Esc "Laden", cEngine::Load from the render loop 0x60e68d) replaces
+all objects, so until the chest reloads on the next tick its ids name the new
+world's objects. A hero saved inside that load (characters.cpp Engine_Load: a
+migrated character's first load makes its world hero the shared hero with
+SaveHero) wrote the shared chest, i.e. those objects, into
+shared_chest.json; the next tick loaded them as the chest. Now the chest is
+emptied and marked unloaded as soon as a save game is read or a game starts
+(RequestLoad), so no save writes it before it is loaded again. The user's
+chest was restored from the 2026-10-19 backup (the broken file kept as
+backup/shared_chest.broken-2026-10-21.json).