GetBestSize() returns an arbitrary hardcoded size for the window, so
you must override it when implementing a custom window class.
+ Notice that the best size returned by this function is cached
+ internally, so if anything that results in the best size changing (e.g.
+ change to the control contents) happens, you need to call
+ InvalidateBestSize() to notify wxWidgets about it.
+
@see @ref overview_windowsizing
@since 2.9.0