Don't do anything in NCPAINT. If you do, you have to do everything. But the
  whole point of DwmExtendFrameIntoClientArea is to let you paint the NC area in
  your normal paint. So just do that dummy.

  * This doesn't transition across monitors.
  * This has a window style change I think is wrong.
  * I'm not sure the margins change is important.

…high DPI

  I'm not entirely sure why. But if we only update the titlebar drag region when
  that actually changes, it's a _lot_ smoother. I'm not super happy with the
  duplicated work in _UpdateDragRegion and OnSize, but checking this in in case
  I can't figure that out.