About the new tab button
I have enabled the option to have no space above the tabs in maximized window mode: it's great, but can't we have the 'New Tab' button to behave the same way? Right now if I hover it with my cursor to the top edge of the screen it turns white, the tooltip even appears, but if i click it it doesn't work: I have to move the cursor down by a few pixels (it really looks like 1px is enough, but its hard to tell...) and there it would react to my click. Just a Quality Of Life change, would help saving some time ^^
It's a known issue.
Ctrl T saves even more time.
Until Vivaldi team applies these mods officially, you can use these
new-tab-button-fix.css is what you're looking for