Change Windows theme from Aero to Classic (or vice versa): Qt windows fail to adopt to window decorations changes.
Typically, Classic theme has thinner borders and title bar. QWidget remains the same size, and it's contents remain the same size. In case of aero->classic switch - empty space appears between content and window frame. In case of classic->aero switch - parts of contents are hidden below frame.

The same happens when changing title bar and borders sizes manually.

See screenshots for aero->classic switching.

WM_THEMECHANGED, WM_STYLECHANGED should be handled properly.