how to detect why I got the focus

[Harry]

I need to detect why my frame has received the focus.
The reason for this is that I have quite a complicated constellation of 
windows positioned within the frame.
If the user clicks by mistake inside the frame, I would like to return 
the focus to his last window.
If the user clicks the frame by intention, for example to resize it, I 
would like to let him do so without interference,
process the resize event, and only afterwards return the focus to his 
last window.

Under MSW I would process the WM_MOUSEACTIVATE message before getting 
the focus to find out the reason why.
However, I haven't found any such mechanism in wxWidgets that allows me 
this functionality.

Anyone has any idea how I might detect why I got the focus in a portable 
way ?