--- C:/Users/User/Desktop/auibook.py	Sun Nov 05 12:15:57 2017
+++ C:/Python27/Lib/site-packages/wx-4.0-msw/wx/lib/agw/aui/auibook.py	Sun Nov 05 12:14:26 2017
@@ -5061,7 +5061,7 @@
                 new_tabs._tabs.SetArtProvider(self._tabs.GetArtProvider().Clone())
                 new_tabs._tabs.SetAGWFlags(self._agwFlags)
 
-                self._mgr.AddPane(new_tabs, framemanager.AuiPaneInfo().Bottom().CaptionVisible(False), mouse_client_pt)
+                self._mgr.AddPane(new_tabs, framemanager.AuiPaneInfo().Bottom().CaptionVisible(False).BestSize(self.CalculateNewSplitSize()), mouse_client_pt)
                 self._mgr.Update()
                 dest_tabs = new_tabs._tabs