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Newest versions of Firefox (e.g. 78.5.0) and Thunderbird (e.g. 78.5.1) are sometimes displayed scrambled (dots, lines, diagonally inclined text and images, black screen, rapidly blinking...).

Steps to reproduce: almost often when resizing the applications and then rapidly sweeping the cursor through them. Also, sometimes this happens when the applications are first shown or when typing text. When the applications get refreshed, they recover a normal aspect (thus, it is impossible to take a screenshot of the artifacts). The same applications are shown normally when running under XWayland. The problem seems independent of the graphic card (it appears with nvidia under nouveau and intel).