When I use openGL compositor, sometimes after a while the screen starts to flicker like there was a double buffer wrong. it flips between black & the real content, for only in parts of the display (a 4K display)

$ inxi -Gxx
Graphics: Card: Intel Device 5926 bus-ID: 00:02.0 chip-ID: 8086:5926
Display Server: x11 (X.Org 1.20.0 ) drivers: intel (unloaded: modesetting,fbdev,vesa)
Resolution: 3840x2160@30.00hz
OpenGL: renderer: Mesa DRI Intel Iris Plus Graphics 640 (Kaby Lake GT3e)
version: 4.5 Mesa 18.1.3 (compat-v: 3.0) Direct Render: Yes

This happens since the beginning I use KDE on Arch, usually what I do is :
- go to display settings -> compositor
- switch from the current OpenGL to the other 3.1 to 2.0 or the opposite
- apply and it disappears most of the time, but after a while, again
- when it doesn't work, I choose xrender

This never happens when I choose Xrender, but it is a pity using xrender with such a good graphic card right?
I never had this annoyance when I was using Maté


EDIT:
I captured a small video to show that, but we can't upload apparently

this forum is useless.
there is no one here ;-(

now with recent version it seems the problem has disappeared.

Bravo

(Intel Iris Plus 640 on HP ENVY 15-as110nf with OpenGL 3.1)
(00:02.0 VGA compatible controller: Intel Corporation Device 5926 (rev 06))