I did a bit more investigation - after the toolbar "freezes", this message shows up in the console:
(Pinta:7362): Gdk-CRITICAL **: IA__gdk_window_thaw_updates: assertion `impl_window->update_freeze_count > 0' failed
I did a bit more investigation - after the toolbar "freezes", this message shows up in the console:
(Pinta:7362): Gdk-CRITICAL **: IA__gdk_ window_ thaw_updates: assertion `impl_window- >update_ freeze_ count > 0' failed