https://gitlab.freedesktop.org/mesa/mesa/commit/30a01cd9232ed83a0259d184b82e050bae219ed3 has to be re-worked to work/to be integrated into with the software renderer as well.
Forget to add https://gitlab.freedesktop.org/mesa/mesa/merge_requests/818 as related.
Apologies, the commit that fixes the problem is actually 903ad59407ac965f9fbc8c0c397cc6f09263a2b8: not the one that I posted initially (it fixes that actually).
Giving this a quick stab, is this what you mean?
This seems to happen only when resizing the window before having a change to flip the back/current buffers. Pushed I fix which works but not sure if fixes the problem at the correct level: