Fix menubar focus using Alt on Windows (#2521)

CHANGELOG.md

@@ -8,6 +8,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 
 # Unreleased
 
+- On Windows, fix focusing menubar when pressing `Alt`.
 - On MacOS, made `accepts_first_mouse` configurable.
 - Migrated `WindowBuilderExtUnix::with_resize_increments` to `WindowBuilder`.
 - Added `Window::resize_increments`/`Window::set_resize_increments` to update resize increments at runtime for X11/macOS.

src/platform_impl/windows/event_loop.rs

@@ -1537,7 +1537,11 @@ unsafe fn public_window_callback_inner<T: 'static>(
                     },
                 });
             }
-            0
+            if msg == WM_SYSKEYUP {
+                DefWindowProcW(window, msg, wparam, lparam)
+            } else {
+                0
+            }
         }
 
         WM_LBUTTONDOWN => {