When it occurs, I can still Shift P to Pause a task and use the action buttons along the top of the list of tasks

Hi clivey,
This sounds like a bug we're looking into with Chrome and the desktop app.

If you know any conditions that cause this to happen, that'd help to know—e.g. "this only happens right after I start my computer" or "this happens when I select this list".
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In the meantime, this bug we're tracking only seems to be happening in the Chrome JavaScript engine, so it would only affect Chrome and the desktop app. If you wanted to use the web app in another browser in the meantime to avoid it, that should work.
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​Keep us posted if you find anything else out!

Thanks Andrewski. One time I believe it happened yesterday was when I moved to another list, and then back to my usual Smartlist. However I wasn't able to replicate that, and not long after I posted the message, the Mac app seems to be behaving normally again.
I'll keep you posted if I can add any more, but good to know you're tracking it.

No re-occurrence of this issue for many days now, although I haven't changed anything my end :-)

I'm glad to hear it hasn't come back in a while! I'm not sure if the development team had rolled out any changes in that time, but I'll check with them to keep them updated.

Thanks for keeping us posted!