Tested on Linux build 2000070710. If you open the download dialog and the list of files is longer than can fit in the list the scrollbar won't show up until you scroll beyond the last visible entry. This means that you can't use the mouse to scroll until after you have already used the keyboard. The scrollbar should always be visible if the file list is longer than the window can hold.
*** This bug has been marked as a duplicate of 43467 ***
verified duplicate (of the more general scrollbars missing in trees bug).