Which is fantastic for some use cases, but not for the use case where I want a fully featured browser window in my editor to preview my page i'm currently working on as I'm working on it.

Why not have one's actual web-browser as an adjacent window, with livereload?

I think it's an anti-pattern for the primary application in use to be reflexively maximised and lay claim to the whole screen, meaning everything must be inside it.

If i'm working on a laptop with only one screen, an adjacent window starts to crowd the UI pretty heavily. Having it in a tab is easier for me.

You can call it an anti-pattern, but for me the anti-pattern is alt+tabbing back and forth to see if my changes look good, or cramping the browser to 1/2 the screen and the whole editor (with sidebar, file tree, and other crap) cramped in the other half, or needing to navigate to localhost:3000 every time I want to look at something VS having a keybinding to open to the exact page i'm working on.

Having a web browser embedded in your editor somehow increases your screen real estate? How do you change from editing to viewing in a different tab that is any different from changing from editing to viewing in a different window?

True.

Still doesn't change the fact that you can open the page in a normal browser and autoreload it with some pluging in other text editors, but you can't have the same performance in Atom ¯\_(ツ)_/¯