Fullscreen YouTube playback enabling G-Sync
When watching YouTube in full screen G-Sync appears to be (likely unintentionally) enabled, this can cause videos to 'flicker' especially while paused, a side effect of the browser not redrawing frequently enough to keep colours stable. This can be easily verified by turning on the G-Sync Indicator (in Nvidia Control Panel top menu, Display -> G-Sync Indicator).
One way to 'fix' this to set the GSYNC - Application State to 'Disallow' within Nvidia Inspector for the Vivaldi exe, however Vivaldi developers may want to investigate a better solution as allowing video to playback using G-Sync would improve playback smoothness for all video frame rates if done correctly. Alternatively, alerting Nvidia to this issue may get them to fix it.