Better Inspect Element
In the current version of Opera and also the current version of Chrome when you inspect an element you can also change it to a mobile resolution. You can also chose to stick the inspect element thing somewhere in your browser instead of a pop up. It looks like this in the current version of Opera: http://i.imgur.com/mocmBOH.png I think that these features should also be added to Vivaldi as they would be pretty useful for development purposes.
This button is missing yet, a known Vivaldi bug. VB-11052 "Developer Tools not up to date"
I'll ping a developer.