I just had a very quick look (no time for proper debugging and
fixing the problem right now, sorry), but I think you can also reproduce
this behavior on any other instance of Hatari, too: It happens if there
is another event between mouse button down and mouse button up (which
seems to happen automatically on Miro's system, but normally does not
happen for everybody else). So you can go to the file selection dialog,
push the mouse button down over one of the buttons there, then provoke a
different event like pressing the SHIFT key, and then just release the
mouse afterwards. The button then stays pressed.