I made some tests with kmsgrab (I have used the vaapi interop) and it
seems to me that sometimes if an app (e.g.: ffplay) is in fullscreen it
does not capture a fully drawn framebuffer but the one which is being
drawn to and which has not been presented to the user.
Also sometimes the capture stops with errors like this:
Until the app is in a window, everything seems to work. If I switch it to
fullscreen, it will fail sooner or later. Happens with a 4.2 release as
well. I used the lightweight nodm window manager for testing. Happens with
ICEWM as well.