I use a tiling window manager, meaning all windows combined take up 100% of the screen, and all of them are always visible. The only UIs that never handle this correctly are modern, responsive websites which sometimes become unusable and show baffling design decisions at certain window sizes.

Although in the case of Reddit the old design isn't perfect either, because if the browser window is narrow enough it'll have a bug where you can shift the whole website out of the visible area by writing a long line in a comment.

To be clear, I know this is not a trivial thing, but when UI designers don't know how to handle certain viewport sizes they should rather just let the browser's scrollbars do the job they've been doing fine for decades.

Yeah, it feels weird. I haven't maximized a window in a long long time. Well unless watching a video counts.