Bug 385692

I have the issue Marco described on the mailing list.

Symptoms:
 - open a big file in kate
 - scroll quickly
 - flickers like crazy

This happens on kate, but never gedit
Marco says this doesn't happen in Weston
This does happen on neseted wayland sessions
It doesn't happen on testRenderingServer in kwayland (which is SHM based)

I'm pretty confident the real bug is in the QPA, but I'll track it here.

We can see in wayland debug:

[2180114.022]  -> wl_surface@40.frame(new id wl_callback@226)
[2180114.066]  -> wl_surface@40.attach(wl_buffer@160, 0, 0)
[2180114.208]  -> wl_surface@40.damage(529, 40, 2, 438)
[2180114.255]  -> wl_surface@40.damage(33, 478, 498, 2)
[2180114.303]  -> wl_surface@40.commit()
[2180114.324]  -> wl_surface@40.frame(new id wl_callback@139)
[2180114.360]  -> wl_surface@40.attach(wl_buffer@160, 0, 0)
[2180114.405]  -> wl_surface@40.damage(33, 38, 498, 2)
[2180114.442]  -> wl_surface@40.damage(33, 40, 2, 438)
[2180114.492]  -> wl_surface@40.damage(529, 40, 2, 438)
[2180114.538]  -> wl_surface@40.damage(33, 478, 498, 2)
[2180114.585]  -> wl_surface@40.commit()
...
[2180120.358] wl_buffer@160.release()
[2180120.378] wl_buffer@160.release()


This looks to me like it's attaching the same buffer twice (and in some cases 4/5 times) before getting the release of the buffer

We could be reading those pixels after the first commit; hence flicker