I'm using Workstation 15.5.6 build-16341506 in Windows 7. I've noticed that if I full-screen (ctrl-alt-enter) a Windows 10 guest and then exit full screen (ctrl-alt-enter) the pane that shows all the guests is slightly smaller. As a workaround I can restart Workstation while leaving the VMs running in the background, and the size goes back to normal. This is a problem for me because I do not use auto-fit for my guests so when the pane is sized slightly smaller then there are scrollbars on it, making it necessary for me to use the workaround.
It appears to be some sort of widget issue where the border goes from thin to thick by adding an extra 2 pixels. Here's a before & after comparison:
My display DPI is set to 125%, which may have something to do with it. I don't know how long this has been an issue since I rarely use full screen mode.