With each relaunch of the Editor, the Main Frame Window grows by 10x10 pixels.
The editor viewport, in turn, grows by a few pixels in response to that.
See attached three images: Window1, Window2, Window3 Each window was screen captured at 100% with Snatch-it showing a progression of 10 pixels larger in diameter each time the editor was opened.
Not a regression: Occurs in UE4.14.3
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-42097 in the post.