Partial workaround for stale styling information with GTK3

We can trigger size events when we know the style cache has been updated.
See #16088

src/gtk/toplevel.cpp

@@ -1020,9 +1020,7 @@ bool wxTopLevelWindowGTK::Show( bool show )
         // size_allocate signals occur in reverse order (bottom to top).
         // Things work better if the initial wxSizeEvents are sent (from the
         // top down), before the initial size_allocate signals occur.
-        wxSizeEvent event(GetSize(), GetId());
-        event.SetEventObject(this);
-        HandleWindowEvent(event);
+        SendSizeEvent();
 
 #ifdef __WXGTK3__
         GTKSizeRevalidate();
@@ -1031,6 +1029,14 @@ bool wxTopLevelWindowGTK::Show( bool show )
 
     bool change = base_type::Show(show);
 
+#ifdef __WXGTK3__
+    if (m_needSizeEvent)
+    {
+        m_needSizeEvent = false;
+        SendSizeEvent();
+    }
+#endif
+
     if (change && !show)
     {
         // make sure window has a non-default position, so when it is shown
@@ -1325,15 +1331,20 @@ void wxTopLevelWindowGTK::GTKUpdateDecorSize(const DecorSize& decorSize)
         // gtk_widget_show() was deferred, do it now
         m_deferShow = false;
         DoGetClientSize(&m_clientWidth, &m_clientHeight);
-        wxSizeEvent sizeEvent(GetSize(), GetId());
-        sizeEvent.SetEventObject(this);
-        HandleWindowEvent(sizeEvent);
+        SendSizeEvent();
 
 #ifdef __WXGTK3__
         GTKSizeRevalidate();
 #endif
         gtk_widget_show(m_widget);
 
+#ifdef __WXGTK3__
+        if (m_needSizeEvent)
+        {
+            m_needSizeEvent = false;
+            SendSizeEvent();
+        }
+#endif
         wxShowEvent showEvent(GetId(), true);
         showEvent.SetEventObject(this);
         HandleWindowEvent(showEvent);