So, win7 user here, noticed that game viewport shrinks when using certain special attacks (like lightbringer). HUD does not shrinks, only game itself. After it shrinks it gradually goes back to normal in seconds. Looks something like this: https://i.imgur.com/YlLauUb.jpeg
Possible fix: turn off "FPS priority mode" in Graphic Settings menu. Worked for me.
I assume, in this mode, game tries to lower resolution at times when a lot of effects happen, but because of some shenanigans (be it old unsupported OS, bad video drivers or bad game code itself) it dosent work as intended, changing size of whole viewport instead.