Auto-complete URL bar
I get the feeling this will be perceived as a minor problem, but it drives me crazy when I type a search term in the URL bar. If I make a mistake but Vivaldi auto-completes the search term to a URL from the history, [b]pressing Backspace doesn't remove the last character I typed[/b], it just undoes the auto-complete. This often results in a misspelled search because I didn't realize my Backspace key-press wasn't properly applied. Maybe instead of auto-complete where the text is selected, you could first make people press Tab (or something) to activate the suggestion? Firefox, for example, makes you press Tab to cycle through the suggestions.
I agree; that shouldn't happen. The current mechanism for autocomplete (in which Vivaldi adds text and range-selects it so that further user typing replaces it) also messes up the Undo command, as I recently noted.
Like I said in that thread, I think the completion segment should be some faded text that shouldn't actually be added to the functioning input text until you do something like mouse-click on the suggestion, or press the right arrow key on your keyboard when the text cursor is positioned at the rightmost end of the existing functional text.
Your suggestion of using the tab key made it occur to me that the ability to tab through multiple completion possibilities (similar to the CLI tab completion that is common now) would be useful.