core/window: ensure items are polished before setting window visible

Hacks around a bug in layouts that commonly results in popups being
wrongly sized for at least a frame.

src/core/proxywindow.hpp

@@ -133,5 +133,6 @@ protected:
 	bool reloadComplete = false;
 
 private:
+	void polishItems();
 	void updateMask();
 };