Windows Resizer extension
I'm using Vivaldi 1.5.658.21 (Official Build) (32-bit), Win10 Enterprise 64bit.
Yea, I guess Vivaldi has changed enough under the hood that these don't work anymore.
My guess is there was enough rewrite, making it different enough that breaks these extensions.
Not sure there much one can do other than write a whole new extension specifically for Vivaldi