I don't think that's it.  If I hold down Alt deliberately
and press Tab, the silly Alt-Tab popup hangs around.
When Tab was misbehaving, I had no popup.

I think this is just some Windoze stupidity.  Who knows.
I killed an IE process earlier today from the Task Mangler,
maybe that messed something up.  Regardless, I'm
adding the Alt-Tab Banishing Ritual to the Grimoire :-)