Keyboard shortcuts for web-pages trigger browser shortcuts instead!
I have Vivaldi 1.7.735.46 on Linux. I have a most annoying problem where the keyboard shortcuts for Vivaldi trigger when, arguably, they should not. The examples I have both revolve around web-based email or messaging apps, where pressing keys like "backspace" or "ctrl-b" cause Vivaldi to react rather than just the email app. The "backspace" one is the worst because it takes you back in the history, losing everything that's just been typed.
What is interesting is that the web app still does get the keystroke and acts on it, but that then Vivaldi does its thing on top too. So, pressing "ctrl-b" really does set the text entry to bold in the email app, but I also get the bookmark page which I then have to close before I can continue.
As a work-around I have simply disabled Vivaldi's shortcuts entirely, but it is a shame to have to do so. FWIW, Chrome and Firefox don't exhibit this behaviour, so maybe it is simply that Vivaldi doesn't properly recognise when a web-page is taking its own input.
The problem of bleeding shortcuts from webapp into Vivaldi is in progress.
Soem problems are fixed in 1.8 Snapshot, seom not.
You can disable/change the disturbing shortcut in Vivaldi's keyboard settings.