Created attachment 143784 [details]
Demo of the problem

* Background

compton has the ability to do partial screen update. It will gather the damaged
regions from Xorg, query the buffer age using GLX, and then only update the
part of the screen that is damaged.

* Problem

When partial screen update is enabled, sometimes closed window is left on the
screen. 

Please see the attachment. 

In the clip, at first, I was repeatedly opening/closing Rofi, trying to
triggered bug (it does not reproduce consistently). After several attempts,
Rofi window is left on the screen after I closed it. Then, I moved my mouse
around to trigger updates of the window underneath.

However, if I capture a trace of this happening with apitrace, this problem
doesn't happen during the replay of the trace.

* Thoughts

Could it be mesa is returning the wrong buffer age of the back buffer?

* To reproduce

Build compton from the repo. Run it with `compton --config /dev/null
--use-damage --experimental-backends --backend glx`. Then try open/close window
several times.

* System info

Card: Radeon RX590
DDX: xf86-video-amdgpu
Mesa version: 19.0.0