Our client size calculation can be wrong for reasons beyond our control, notably on Ubuntu 12.04 with overlay scrollbars where the GtkScrolledWindow "scrollbar-spacing" is wrong. By using the true size from size-allocate when it becomes available, we can recover from an earlier incorrect client size calculation. See #14871 git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73231 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775