The tag panel can be attached to the top of the mp3tag window and then consists of several columns, where the last column may contain cover art:
If I now turn the display of the cover art off, the horizontal scrollbar vanishes too and mp3tag shows only the left part of the columns of the tag panel:
Now it's not possible to scroll to the last column of the tag panel any more. Only resizing the main window or the tag panel helps to bring back the rightmost columns into sight, but nothing of these measures lets the scrollbar reappear, except of course turning display of cover art on again.