the answer was so dumb and so simple, I could only laugh in disbelief at the rabbith holes this thing had gone down looking for the answer --
It's an over-engineered Select (yes I know, but it's a good headless library), I had forgotten to deal with the situation when the number of options made the list longer than the popover trying to contain it. The options listbox then overflowed the button that opened it.
On click, the mouse-down opened the listbox, the mouse-up immediately chose a value.
