@BunxBun I can change the UI back to normal on Vivaldi 6.7.3329.17 (Stable channel) (64-bit)]. The path is:
Vivaldi Menu > Settings > Appearance > MENU > [checkbox] Compact Layout.
Or search for compact layout.
Unfortunately the trend on PC user interfaces is to introduce a touchscreen like UI because we all spend so much of time with smartphones. Companies think a similar UI on different devices would be something good.
In my opinion, a personal computer and a phone are simply different. I use a mouse and a keyboard as input for my PC, why would I like everything being way too big for no good reason is beyond me.
I wish they didn't change my UI without permission after I updated Vivaldi. Just a heads up on the What's New page would be more than enough for users to acknowledge the new option. Although, I'm glad they didn't just ripped away the compact mode.