Ah, sorry, I misunderstood your comment.

I have applied the customization you suggested, which effectively disabled the "open new window" function.  Is the idea that I should apply this customization when the odd behavior occurs, as a test?  Or are you suggesting that I should use this to disable shift-click as a long-term workaround to the problem?

The reason I said it was something happening in the tweaker is because the problem always goes away after I restart the tweaker.  So at the least, the tweaker is resetting something.

I guess the thing I should test the next time the issue occurs is: what happens if I close the tweaker, but do not restart it.  Does the issue persist, or does it go away when the tweaker is closed?

I see that there is no existing customization for this item, which is what I was looking for.  I thought it might be getting set in software but I guess that's not what is happening.

I can assure you that it is not a matter of the key getting physically stuck.  When the issue occurs, it persists until I restart the tweaker, then goes away as soon as I do so.  So it's 100% happening in software, and is related to something that is happening in the tweaker.

The behavior I have encountered does in fact match the behavior of a shift-click, but I assure you that I am not shift-clicking.

When this occurs, it is very persistent and happens with all items in the taskbar (whether they were already open or not) and keeps happening no matter how many times I click.

As soon as I kill and restart the tweaker, the behavior vanishes.

Unfortunately, this is a heisenbug.  It shows up randomly with no clear pattern, though now that I think about it I think it has only happened after resuming from sleep.

Notably it seems to be happening much more often in the last month or two, so maybe it's related to something that changed in the last update?

In any case, if there is some kind of log or other report I can send, I'm happy to do so.

If you can tell me what "advanced option" setting would trigger this behavior, I can take a look at that setting and see if it is engaged, or if it gets engaged somehow on its own when the issue occurs.

Any other ideas?  I realize this is not a lot to go on, but I'm not sure what else I can provide.