sdl2: Work around SDL2 SDL_ShowWindow() bug

Apparently it is possible for X to send an event to a hidden SDL2
window, leading to SDL2 believing it is now shown. SDL2 will pass the
SDL_WINDOWEVENT_SHOWN message to the application without actually
showing the window; the problem is that the next SDL_ShowWindow() will
be a no-op because SDL2 assumes the window is already shown.

The correct way to react to SDL_WINDOWEVENT_SHOWN would be to clear
scon->hidden (analogous for SDL_WINDOWEVENT_HIDDEN). However, due to the
window not actually being shown, this will somehow not be correct after
all.

Therefore, just hide the window on SDL_WINDOWEVENT_SHOWN if it is
supposed to be hidden (and analogous for SDL_WINDOWEVENT_HIDDEN).
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/sdl2.c

@@ -548,6 +548,16 @@ static void handle_windowevent(SDL_Event *ev)
             qemu_system_shutdown_request();
         }
         break;
+    case SDL_WINDOWEVENT_SHOWN:
+        if (scon->hidden) {
+            SDL_HideWindow(scon->real_window);
+        }
+        break;
+    case SDL_WINDOWEVENT_HIDDEN:
+        if (!scon->hidden) {
+            SDL_ShowWindow(scon->real_window);
+        }
+        break;
     }
 }