windows: fix freeze on keyboard layout switch

Any winit app on Windows freezes when the keyboard layout is switched by
tools such as Punto Switcher. A minidump of the frozen process shows the
main thread blocked in MsgWaitForMultipleObjectsEx (winit's event-loop
wait), re-entered during message dispatch through the switcher's injected
global hook (pshook64.dll, via CallNextHookEx) — a re-entrant Win32
message-loop wait, not a mutex deadlock (no LAYOUT_CACHE frames present).

Handle WM_INPUTLANGCHANGE to refresh the cached keyboard layout, then
defer to DefWindowProc so the message still propagates to first-level
child windows as the Win32 docs require. The cache refresh is the minimal
change that stops the freeze (verified on Windows 11 by isolation
variants); update_modifiers and swallowing the message are not needed.

winit-win32/src/event_loop.rs

@@ -51,12 +51,12 @@ use windows_sys::Win32::UI::WindowsAndMessaging::{
     SystemParametersInfoW, TranslateMessage, WHEEL_DELTA, WINDOWPOS, WM_CAPTURECHANGED, WM_CLOSE,
     WM_CREATE, WM_DESTROY, WM_DPICHANGED, WM_ENTERSIZEMOVE, WM_EXITSIZEMOVE, WM_GETMINMAXINFO,
     WM_IME_COMPOSITION, WM_IME_ENDCOMPOSITION, WM_IME_SETCONTEXT, WM_IME_STARTCOMPOSITION,
-    WM_INPUT, WM_KEYDOWN, WM_KEYUP, WM_KILLFOCUS, WM_LBUTTONDOWN, WM_LBUTTONUP, WM_MBUTTONDOWN,
+    WM_INPUT, WM_INPUTLANGCHANGE, WM_KEYDOWN, WM_KEYUP, WM_KILLFOCUS, WM_LBUTTONDOWN, WM_LBUTTONUP,
-    WM_MBUTTONUP, WM_MENUCHAR, WM_MOUSEHWHEEL, WM_MOUSEMOVE, WM_MOUSEWHEEL, WM_NCACTIVATE,
+    WM_MBUTTONDOWN, WM_MBUTTONUP, WM_MENUCHAR, WM_MOUSEHWHEEL, WM_MOUSEMOVE, WM_MOUSEWHEEL,
-    WM_NCCALCSIZE, WM_NCCREATE, WM_NCDESTROY, WM_NCLBUTTONDOWN, WM_PAINT, WM_POINTERDOWN,
+    WM_NCACTIVATE, WM_NCCALCSIZE, WM_NCCREATE, WM_NCDESTROY, WM_NCLBUTTONDOWN, WM_PAINT,
-    WM_POINTERUP, WM_POINTERUPDATE, WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SETCURSOR, WM_SETFOCUS,
+    WM_POINTERDOWN, WM_POINTERUP, WM_POINTERUPDATE, WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SETCURSOR,
-    WM_SETTINGCHANGE, WM_SIZE, WM_SIZING, WM_SYSCOMMAND, WM_SYSKEYDOWN, WM_SYSKEYUP, WM_TOUCH,
+    WM_SETFOCUS, WM_SETTINGCHANGE, WM_SIZE, WM_SIZING, WM_SYSCOMMAND, WM_SYSKEYDOWN, WM_SYSKEYUP,
-    WM_WINDOWPOSCHANGED, WM_WINDOWPOSCHANGING, WM_XBUTTONDOWN, WM_XBUTTONUP, WMSZ_BOTTOM,
+    WM_TOUCH, WM_WINDOWPOSCHANGED, WM_WINDOWPOSCHANGING, WM_XBUTTONDOWN, WM_XBUTTONUP, WMSZ_BOTTOM,
     WMSZ_BOTTOMLEFT, WMSZ_BOTTOMRIGHT, WMSZ_LEFT, WMSZ_RIGHT, WMSZ_TOP, WMSZ_TOPLEFT,
     WMSZ_TOPRIGHT, WNDCLASSEXW, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW,
     WS_EX_TRANSPARENT, WS_OVERLAPPED, WS_POPUP, WS_VISIBLE,
@@ -1508,6 +1508,21 @@ unsafe fn public_window_callback_inner(
             result = ProcResult::Value(unsafe { DefWindowProcW(window, msg, wparam, lparam) });
         },
 
+        WM_INPUTLANGCHANGE => {
+            // Refresh the cached keyboard layout for the newly activated input
+            // language. This message is sent (by Windows or by layout switchers
+            // such as Punto Switcher) after the layout changes. Refreshing the
+            // cache here prevents a freeze that otherwise occurs when switching
+            // layout via such tools. We still defer to `DefWindowProc` so the
+            // message keeps propagating to first-level child windows, as the
+            // Win32 documentation requires.
+            {
+                let mut layouts = LAYOUT_CACHE.lock().unwrap();
+                layouts.get_current_layout();
+            }
+            result = ProcResult::DefWindowProc(wparam);
+        },
+
         // this is necessary for us to maintain minimize/restore state
         WM_SYSCOMMAND => {
             if wparam == SC_RESTORE as usize {

winit/src/changelog/unreleased.md

@@ -58,6 +58,10 @@ changelog entry.
 
 ### Fixed
 
+- On Windows, fix a freeze that occurs when the keyboard layout is switched by
+  tools such as Punto Switcher. The `WM_INPUTLANGCHANGE` message is now handled
+  to refresh the cached keyboard layout, while still deferring to
+  `DefWindowProc` for normal propagation.
 - On Redox, handle `EINTR` when reading from `event_socket` instead of panicking.
 - On Wayland, switch from using the `ahash` hashing algorithm to `foldhash`.
 - On macOS, fix borderless game presentation options not sticking after switching spaces.