WindowServer: Clear the current resize candidate when in menus

While the menu system is swallowing mouse events, just clear any resize
candidate we had set. This ensures that we don't end up with a resize
cursor when slipping into a menu-controlled part of the screen.

Fixes #1306.

Servers/WindowServer/WindowManager.cpp

@@ -767,6 +767,7 @@ void WindowManager::process_mouse_event(MouseEvent& event, Window*& hovered_wind
 
     // FIXME: Now that the menubar has a dedicated window, is this special-casing really necessary?
     if (MenuManager::the().has_open_menu() || (!active_window_is_modal() && menubar_rect().contains(event.position()))) {
+        clear_resize_candidate();
         MenuManager::the().dispatch_event(event);
         return;
     }