FWIW, the corruption doesn't occur on my laptop with Intel graphics (w/
hardware accelerated compositing, just like on my desktop). Also, on my desktop
when I set LIBGL_ALWAYS_SOFTWARE=1 and the flag in chrome://flags to force
hardware acceleration, I get the same result as Jose (no corruption).

I realized that, at least for me, this is much easier to reproduce on some
pages than on others. It's much more likely to happen with multi-line tooltips.
To make it easy for anybody to test, I exported this HTML page from my web IRC
client: https://www.dropbox.com/s/8kkanknv25jzvtk/test_tooltips.html?dl=1

The easiest way to test it is to mouse over the #radeon channel topic at the
top of the page and view the tooltip (this one is 3 lines long). Then, mouse
over some of the channel names on the left-hand side to view their tooltips.
Then, start mousing back and forth between the channel topic and the different
channels. After a couple of tries, you'll start to see the corruption and stale
content, with parts of the #radeon channel topic being replaced by the other
channel names.