There's another set of fixes which can cause the flickering (Bug 1686703). Vlad, right now we use the damage region to mark updated areas while parts outside the damaged region can be invalid. If KWin does not support that and always needs fully up-to-date wl_buffer submitted I can do that hack for Kwin only.
Bug 1685263 Comment 30 Edit History
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There's another set of fixes which can fix the flickering (Bug 1686703). Vlad, right now we use the damage region to mark updated areas while parts outside the damaged region can be invalid. If KWin does not support that and always needs fully up-to-date wl_buffer submitted I can do that hack for Kwin only.