I use the classic graphics engine and generally have the viewport display resolution set to high although the border appears whether its set to low, medium or high.
Okay - this has been driving me absolutely nuts today - so I resolved to get to the bottom of it after work.
So after a lot of digging I finally found the reason.
I have a border around every Raster Rendered Viewport because somehow I accidentally ticked the box ‘Enable Draft Mode’ under Preferences> Performance.
Unticking the box made the borders disappear like magic: Hooray