Mike Christie at en.wp reports:
Intention: Use scrollbar in the link insertion dialogue to view more possible link targets
Steps to Reproduce: Bring up the link dialogue for any word with many possible links -- e.g. "Texas". A scrollbar will appear on the right of the list. Click on the scrollbar to scroll down.
Results: The dropdown of possible links disappears.
Expectations: It should have scrolled down.
[...] It is possible to scroll using the arrow keys.
Web browser Chrome 35.0.1916.114 m
Operating system Windows 7
Reproduced by me with the same setup.