restrict fix from r75668 to GTK3 only, as it might cause flickering

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75670 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/combocmn.cpp

@@ -1391,6 +1391,10 @@ wxSize wxComboCtrlBase::DoGetSizeFromTextSize(int xlen, int ylen) const
     else
     {
         wxComboBox* cb = new wxComboBox;
+#ifndef __WXGTK3__
+        // GTK3 returns zero for the preferred size of a hidden widget
+        cb->Hide();
+#endif
         cb->Create(const_cast<wxComboCtrlBase*>(this), wxID_ANY);
         if ( m_font.IsOk() )
             cb->SetFont(m_font);