In Javascript terms, something is pulling "focus" out of the entry box, so the keyboard goes away.
I'm not entirely convinced that this is the current issue. It feels like it happens when one of the ad modules reloads. I used the Inspector to hide the reloading ads (display: none) and it stopped doing it.
The smart play (for now) would be for them to kill the reload scripts until they can figure out which one is the problem. (The really smart play would be to modify the reload script to pause while the focus is in the entry field - assuming that's remotely possible.)
It's bad play to reload ads off-screen, anyway - it murders your CTR, which makes it less likely for Google to serve good-paying ads. (I'm out of that biz now, but at one point in time, reloading ads off-screen would have been an AdSense TOS violation anyway.)
Most of the ads I'm getting now feel like dumpster-tier fill-ins. Seriously, like 90% of the ads I'm getting are for whatever this is (and it's unblockable):
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