Xwayland occasionally shows outdated content

I sometimes see outdated content in XWayland. Haven't found a good reproducer yet, but one example is Thunderbird, where e.g. the sender or the title of a mail is not updated or jumps back and forth between the actual and the previous one randomly, sometimes very fast. I assume this is regression from !316 (merged)

Apparently under certain circumstances not all buffers are updated completely.