I have a very similar issue with the Chrome extension on OSX 10.13.6 High Sierra (MacBook Pro). I see this red area when I move from my laptop screen to my thunderbolt display with higher resolution. I have a different problem when moving from the thunderbolt display to my lower resolution laptop screen in that the Enpass window is overly large. It seems that the Enpass plugin isn't adjusting for any (transient) change in screen resolution. I've found that killing and restarting Enpass fixes the resolution issue - but it's a pain as a work-around.