scenario:
1) I play a video full screen with mpv, then quit the player
2) Then I let the machine idle until the screensaver (configured as just
  blanking) kicks in
3) I rouse X with keyboard or mouse
4) The screen turns on as expected, but instead of displaying the expected
windows, it shows the last frame that was displayed by the (now defunct)
video player.  The mouse cursor is correctly rendered, though: it moves, and
changes shape according to which window is (or should be) under it.
5) I can restore a correct display with ctrl-alt-F1 ctrl-alt-F7.

The bug won't occur if the video was not played full screen or if I use a
real screensaver instead of blanking.

The bug only occurs after the first time the screen is blanked after the
video is played. It won't happen again until I play a new video.

The bug also occurs if I use my system for a long time between steps 1 and 2.