How to make full screen mode within window size?

Question

Browsers

There is a large diagonal screen, and I want to place several browser windows side by side and some video can be played in several of them.
I would like that when you click on full-screen mode, it would not be completely full-screen, but limited to the size of the current window in which the player is located.
Maybe there is some kind of extension, or a browser specifically for this.
The kde shell is used, maybe there is something you can twist in the window manager?