I want to view multiple tabs at once, in a split-page view where I can scroll on one tab, then mouse-over to another and start independently scrolling on that one. It's probably the key feature I miss from Vivaldi. Is there some insurmountable obstacle in the engine that prevents implementation, or is it stubborn devs?

Tab groups dying in a ditch

Catching up to Opera circa 2006. Opera added this feature in Opera 9, released June 2006.

I still miss the old Opera. The Chromium-based version just isn't the same.

This is a useless feature. Here are some purely UI features that are more important, and exist in Chromium:

• more compact hight, saving space (make browser.compactmode.show official!)
• CSD decorations (_ 🔳 x) in the top right, hitbox at the very edge, f**k GNOME for this
• Tab groups natively in the Tab bar, its the most organic

Apart from that Firefoxes UI is way better than Chromiums and doesnt need to copy anything.

Then work on performance, process isolation etc.