Random focusing out of windows

Hi, I recently started having a problem with my PC. It would randomly put focus on a window that I am not currently using. An example is if I have my browser out and I am playing a video on a seperate video player, the video would go out of full screen mode into maximized screen (minimize, exit button on the top right but still takes the full screen) and then it would bring up the browser as if I had clicked it on the taskbar.
This problem is trivial and I've dealt with it for several months but I need to rename a significant amount of files for work organization and it is now driving me insane. 8/10 times in the middle of renaming a file, it would just cut out of it, as if I clicked elsewhere. I've tried closing every other window and focus on one window, but it still happens with NO other windows open. I've literally gone on task manager and ended the process of every non-essential program and it still happens. Help me save my sanity and tell me there's some simple, obvious way to solve this.