It looks to me like makers of major web browsers generally cater to the lowest common denominator, or slightly less cynically, but effectively equivalently, aim for a large or universal audience. Their design looks almost indistinguishable from what they would come up with operating by the motto "features confuse users".

Vivaldi has it from beginning because von Tetzchner came from Opera, which also did this back in the day. Their pre-chrome version was an MDI application.

Yes, this. The ability to drag and resize them now is just brilliant. I pretty much only use it to view two pages side-by-side, but when you try it with 4 or 5 tabs the resizing is a dream.

I do that with other browsers with multiple windows. Meta+left and meta+right (in cinnamon and many other desktop UIs) automatically place the window in the left and right half of the monitor.

Firefox can do this with an extension

Except doesn't each browser window have full window chrome?

If the idea is to save screen space that's a non-starter.

no. Thats only how the chrome extension works. In FF, its actually a true split.

OmniWeb (circa 2004?)

I have been able to put two browser windows side by side in every operating system I've used since I first used the web in 1998.