win32: fix ime setcontext lparam

Fixes #3893.

src/changelog/unreleased.md

@@ -46,3 +46,4 @@ changelog entry.
 - On X11, fix `set_hittest` not working on some window managers.
 - On X11, fix debug mode overflow panic in `set_timestamp`.
 - On macOS, fix crash in `set_marked_text` when native Pinyin IME sends out-of-bounds `selected_range`.
+- On Windows, fix `WM_IME_SETCONTEXT` IME UI flag masking on `lParam`.

src/platform_impl/windows/event_loop.rs

@@ -1612,9 +1612,9 @@ unsafe fn public_window_callback_inner(
         },
 
         WM_IME_SETCONTEXT => {
-            // Hide composing text drawn by IME.
-            let wparam = wparam & (!ISC_SHOWUICOMPOSITIONWINDOW as usize);
-            result = ProcResult::DefWindowProc(wparam);
+            // IME UI visibility flags are in lparam.
+            let lparam = lparam & !(ISC_SHOWUICOMPOSITIONWINDOW as isize);
+            result = ProcResult::Value(unsafe { DefWindowProcW(window, msg, wparam, lparam) });
         },
 
         // this is necessary for us to maintain minimize/restore state