Computer is Windows 10 v1903 running both Malwarebytes Premium and ESET NOD32 Antivirus.
The behavior occurs with both the Edge browser and Chrome.
The behavior is that the browser window, whether it was set to full screen or not, very rapidly reduces somewhat in size then expands back to its previous size multiple times in succession over a period of several seconds. This behavior is intermittent, but frequent. The behavior occurs regardless of whether or not there is something actively in the browser window, e. g. typing while composing email in Gmail. I've watched it happen while I've been remoted into the computer just passively watching both Chrome and Edge.
In addition to scans from Malwarebytes and NOD32, I've run online scans from from Norton and TrendMicro.
I have not a clue what might be cause this bizarre behavior that's making it very difficult to work on the computer, and would appreciate any suggestions.