document resize knob not invalidated on look change

[axeld]

When using the LookAndFeel test app, change the look from "Document" to "Title".

You'll see that the knob is not visually removed inside of the window frame; it is redrawn when changing in the other direction, though.

I briefly looked into it, and the region is made dirty, but it seems the client doesn't overwrite, maybe because it's clipping hasn't been changed (although the clipping regions seem to be invalidated correctly in Window).