Web+ rendering glitches on dev.haiku-os.org

[Kev]

I'm not sure exactly what minimal case sets this off, but it was certainly reproducible in context (which is https://dev.haiku-os.org/ticket/10736 -- if you click 'reply' at my comment and try it there, you can get the same behaviour.)

0. start at the above link (screenshot 1)
1. try to single-backspace the end of the echo line
2. the whole line (plus a sliver of the next line) disappears (2)
3. 2-3 seconds later (when the preview is done updating) it reappears (3)
4. type a " character, the line disappears again (looks like 2 again)
5. 2-3 seconds later, the line reappears again (looks like 1 again)

hrev47114.