If the MP3tag window size leads to scrollbars in the tag panel and the tab key is used to navigate between the input boxes then the tag panel does not scroll if the input focus moves to an input field lower than the last visible input box.
The input focus becomes invisible.
There does not seem to be any way to get the tag panel to scroll with a keyboard action - one has to use the mouse for scrolling.
My expectation would have been that the tag panel would always scroll in such a way that the input focus stays visible.
Thanks for pointing, I'll look into it.