Starting in Office 2013, Microsoft implemented a new GUI architecture, and Office doesn't respect the theme or personalization settings of the OS. I don't like it, and it causes problems for me quite frequently (the code, not the visuals). It caused problems when I used 2013, and over the years, I've started to figure out parts of the problems.
One of the problems is the transparent border around the window. How do I turn this off?
I usually run windows using the "classic theme", which disables almost all transparency effects. I use Windows 7 Ultimate and Windows 10 Enterprise.
The transparent border is outside of the regular window, so if I want to resize the window, I don't click on the border, like I do with every other window, I have to click outside the window. This is a problem in and of itself, because when I try to adjust the window size, I frequently wind up clicking on the window behind the window I am trying to adjust.
This forum post is related to the phenomenon, and probably two VDI technologies fighting each other. However, I already disable hardware acceleration when I install Office.