Fix X11 decorations for client-drawn Cocoa windows

Synchronize Motif decoration hints on style and titlebar-property changes, including native-window recreation and restoring ordinary WM decorations. Add a ten-case standalone Cocoa/X11 regression probe and document validation on KWin/Xwayland.

docs/TESTING.md

@@ -119,6 +119,33 @@ Expected: `CORETEXT_FONT_LIST pass=1` with nonzero family/face counts (306/1124
 
 The font probe reproduces Chromium's installed-family enumeration, mandatory family matching and localized-name lookup with a null language output. It checks ownership of the single-match result and that a nonexistent mandatory family produces no substitute. The resize probe checks backing-layer bounds immediately after frame/bounds changes, including a scaled view and a child layer, without relying on input or coordinate conversion to refresh them. It also verifies that native parent size is current during child layout, and that adding an overlay sibling after the content view does not redirect subsequent content resizing into that overlay. The old sibling selection left content at 1600×900 while stretching the 30×20 overlay to 1280×720; the corrected layout resizes the content and preserves the overlay. Before the fixes, these checks report a stale 1280×720 layer and native parent after a 1600×900 resize. The `FRAME_SIZE_HOOK` check uses a subclass whose `setFrameSize:` publishes its viewport size. It checks growth and shrinkage through parent autoresizing, direct `setFrame:`, and direct `setFrameSize:`. Before component-setter dispatch was corrected only the final direct size setter reached the subclass (one callback); afterward all four updates do, with current geometry visible inside the callback. No probe launches or contacts Riot/League. The user confirmed that the language dropdown and League window resizing work after the final X11 correction (2026-10-15). The practice-tool crash remains unresolved.
 
+### X11 window decorations
+
+`python3 tools/build-riot-x11-runtime.py` also builds
+`build/riot-window-decorations-check`. Install and activate both the patched
+AppKit and X11 backend before running it, with clients and the game closed:
+
+```sh
+darling shell env DYLD_LIBRARY_PATH=/Users/me/RuntimeLibraries \
+  DYLD_FRAMEWORK_PATH=/Users/me/RuntimeFrameworks \
+  /Volumes/SystemRoot/path/to/darling-lol/build/riot-window-decorations-check
+```
+
+Expected: `WINDOW_DECORATIONS_CHECK cases=10 pass=1`. The independent Cocoa
+probe reads `_MOTIF_WM_HINTS` on its own unmapped native windows. It covers a
+normal titled window, hidden title alone, hidden plus transparent titlebar,
+both reverse transitions, untitled masks with additional flags, switching to
+borderless after creation, native-window recreation, and properties set before
+the native window exists. It sends no input, maps no windows and does not
+launch or contact Riot/League. Check the success marker, not only the launcher
+exit status: some Darling versions do not propagate a guest assertion failure.
+
+Before the fix the hidden/transparent case fails. With the corrected overlays,
+the ten cases passed on KDE Wayland/Xwayland. Passive inspection of the reopened
+Riot Client also confirmed the no-decoration hint and identical compositor frame
+and client dimensions (1536×864), instead of the previous 28-pixel titlebar.
+This validates the Riot Client window, not every LeagueClientUx or game mode.
+
 ### Presentation publisher handoff
 
 With clients closed, run `build/riot-client-resize-check --handoff` through Darling with the normal runtime overrides. This creates an owned window and two fixture frame slots. The live software owner first supplies a 1280×720 main frame, then switches to a 262×131 popup; another slot supplies the resized main frame without the software flag. The test samples its own window beyond the original frame extent and checks generation colors across growth and shrinkage. The old selector reports black at the 1600×900 sample; the correction passes all four sizes. Repeat with `RCLOL_XSHM=0` and with `DARLING_XSHM_LIBRARY` pointing to the normal transport to cover both paths. The fixtures remove their own frame slots on exit; they do not read client pixels or send input.