What kind of tests would you like to see?
That listview draws correctly after this fix.
Are you questioning correctness of the DefWindowProc(WM_NCPAINT) handler?
No. The MR looks fine at first glance. I just think it would be better to have some tests. It shouldn't be too difficult to test each comctl32 control's WM_NCPAINT handler. I've seen many strange behaviors in comctl32 so I wouldn't be surprised if they don't do what DefWindowProc(WM_NCPAINT) does. I also don't want to block this. So feel free to remove me from the reviewers if you want.