Created attachment 806 [details]
Example screen shot
Bug 6079 added very basic HiDPI support to the client on macOS. Unfortunately there is a slight graphical glitch in this mode. There seems to be an off-by-one error when updating the screen, leaving a very thin line of the previous data. It's easily reproduced by dragging a selection box around on the desktop (see screenshot).
For now the only workaround is disabling HiDPI for the client. To do this right click on the app and select "Get Info". In the info dialog there is the option "Open in Low Resolution". This will get things back to pre-bug 6079 ways.
We decided to revert this change in bug 7047, so nothing to do here.