Problems with mouseover text

In the last couple of days, I've noticed that my mouseover text -- the kind that every xkcd comic has, for example -- is only showing for a fraction of a second, which isn't long enough for me to read it.

I'm seeing the same thing -- I assume for the same reason -- when I mouse over an emoji to see what it's supposed to be.

Any idea what's causing it, and if there's any way to fix it?