If you type something into a combo box, then quickly tab away, the dropdown can pop up after you've already left the field. Since you're no longer focused on the field, there's nothing to do to cancel it. Hitting escape does cause the list to disappear, but the shadow sticks around.
It's debatable what the right solution is. I assume that if the query comes back with a single result, it might make sense to just assume that it was selected. Not sure how to handle the two-or-more case, I suppose it sort of depends on the forceSelection param. Either way, the pop up should check for hasFocus before showing...
* Another "interesting" feature I found: if you start typeahead, it automatically selects the first item in the list. If you hover over (but don't select) an entry in the list, and then hit tab - that entry will be selected, rather than the text that was previously selected. Strange!
* Finally a request: it would be nice to be able to set a display template for the textfield part of the combo, so that when you select an item from the list, the text field's value can be set to a more complex thing than just a single field's value.