MacOS Mojave: Black dashed lines appearing in Apple apps between side bars and main windows

Just upgraded to Mojave, 2nd day in, and I've got black dashed lines in between the side bar and the main window, on Finder, Calendar and the Messages app. Possibly more but those are the one's I have noticed them. These get better or worse, depending on where i move the window around the screen. I have a 1440p external monitor, but this problem also exhibits on the Macbook Retina screen also when unplugged from the main screen. Suggesting that this is a software issue because this wasn't visible in high sierra.

Macbook Pro mid 2014

Iris Pro graphics