I wrote a powershell for it
Running as administrator is needed to write in C:\Program Files
$file = (Resolve-Path 'C:\Program Files\Vivaldi\Application\*\resources\vivaldi\bundle.js')
Copy-Item -Path $file -Destination (Split-Path $file -Parent | Join-Path -ChildPath 'bundle.js.bak') -Force
(Get-Content -Path $file -Raw) -replace '(div#tabs-tabbar-container"\)&&\(\w)=!0\)', '$1=!1)' |
Set-Content $file