ZDoom

I'm asking for this because whenever I'm in fullscreen, my internet doesn't work. It will disconnect me from steam after a while, and netflix stops working. After that, It just disconnects me from internet all together. And as to why I don't use windowed mode, it's because it cuts off the top of my screen. Sorry for bothering you guys, but I've tried just about everything besides reinstalling windows, which I don't really think would help.

Try Borderless gaming?

It works with GZDoom but it tends to be glitchy from what I've experienced with it. It's great for other older games, though.

Real borderless fullscreen would be nice.

ZDoom worked (and probably still works) flawlessly with Borderless Gaming, though older versions were kinda buggy with GZDoom because it moved the window exactly 50 pixels down (so bottom HUD was cut and there was a black panel on the top of the screen) and you had to tweak it a bit but it's been fixed in recent versions.

This is probably not helpful, but this is completely a non-issue on Linux, since any halfway decent window manager allows you to configure per-window settings in detail.

A "no window border" option would do the trick.

Like the "-noborder" command line parameter and "r_noborder" cvar in Call of duty games, which in combination with setting the in-game resolution the same as your desktop resolution, displays full-screen in a window, it's great for alt tabbing glitch free.