I just installed emacs25 and discovered that it doesn't support
scrollbars. It apears to be reserving space for them, by drawing a light
grey line near the edge of the screen, but the space is empty and
doesn't respond to mouse clicks. `M-x toggle-scroll-bar-mode' just moves
the grey line to the very edge of the window. I tried emacs24 too, it
has the same problem.

reassign 859964 emacs24
forcemerge 790098 859964
affects 790098 emacs25
thanks

It suddenly occurred to me that I should check whether this bug is
hidpi-specific, and it is. Adding hidpi to my search I found this is an
old bug.