Created attachment 376140 [details] XUL test case Occurs when the number of listitems is few and the rows attribute is set to the number of listitems. Attached test case uses 5 rows. When I click the down arrow the scrollbar disappears. When one more listitem is added and the rows attribute is set to 6, works as expected -- no scrollbar appears. Occurs on OS X, Windows, and Linux, although on Linux the test case takes 7 rows to work as expected. I played around with adding 1px of padding to the tops and bottoms of the listitems and this made it work as expected. FWIW, ran into this while working on Firefox's clear recent history dialog, bug 489958 comment 13, but only on OS X and with a different number of rows. The listbox is still usable but it sure is unpolished.
Created attachment 378786 [details] screenshot 2 Not sure if this is related, but the scrollbar in this window is also inaccurate but in a different way. The Takuapa listitem is the last in the listbox, but the scrollbar tells me there are more out of view. There are 8 items total. When I click the down arrow, the items in the listbox jitter up and down. When I click and hold the down arrow, the items jitter up and down very quickly until I release the button.
This is still an issue (which I ran into during add-on development). I'm able to reproduce this on Windows 7 (Aero on) with Firefox 32.0.3. The unnecessary scrollbar appears for listboxes with 2 to 4 rows/listitems on my system. (Just use the initial testcase and adjust the number of rows/listitems accordingly). I was able to work around it setting "overflow:hidden;" on the "xul:listboxbody" that is wrapping the listitems internally (via CSS), but that surely is only a hack and not a real solution.
I can reproduce the unexpected scroll bar since Firefox0.9 at least.
You need to log in before you can comment on or make changes to this bug.