Detected size of GTK windows with headerbar is larger than what it should be

You can try for example GEdit or Pitivi. The detected size has +25 pixels on the left, +25 pixels on the right, +22 pixels at the top, +28 pixels at the bottom. Same result with -n or without.

If you use Alt+PrintScreen in Gnome Shell you get a screenshot of the focused window without these margins, so it should be possible.

Yeah my mistake, didn't read the X11 protocol closely enough.

Could you tell me if https://github.com/naelstrof/slop/tree/slop-release-candidate has the same issue?