The HDMI-connected display on my RPI 4B frequently becomes unresponsive. The
system continues to run, however, and using ssh to connect and "sudo systemctl
restart display-manager" will restore the display (after logging the session
out, of course). These occurrences are accompanied by the following log
records.

  Jun 16 10:14:10 rpi4a.WalkerStreet.info kernel:
[drm:drm_atomic_helper_wait_for_dependencies [drm_kms_helper]] *ERROR*
[CRTC:66:crtc-3] flip_done timed out
  Jun 16 10:14:10 rpi4a.WalkerStreet.info kernel:
[drm:drm_atomic_helper_wait_for_flip_done [drm_kms_helper]] *ERROR*
[CRTC:66:crtc-3] flip_done timed out
  Jun 16 10:14:20 rpi4a.WalkerStreet.info kernel:
[drm:drm_atomic_helper_wait_for_dependencies [drm_kms_helper]] *ERROR*
[PLANE:62:plane-3] flip_done timed out
  Jun 16 10:14:30 rpi4a.WalkerStreet.info kernel:
[drm:drm_atomic_helper_wait_for_dependencies [drm_kms_helper]] *ERROR*
[PLANE:153:plane-25] flip_done timed out

Any ideas?