Have you tried clicking on the "fullscreen" icon on the right top when you press tab?
When did you start having this problem?
Try switching to a different browser and see if the same problem happens there.
If it's happening on every browser then it's probably something else. It used to happen to me in Unity, for me, changing the screen resolution of my display used to work, but it's just a temporary solution.
Clear your browser cache (CTRL+Shift+Del) or altogether of your computer by using a cleaner.