Hello,
Going from your post it looks like you’re having some kind of video graphics issue.
I looked up some guides and posts regarding users having a similar issue, looks like you already went for what most others did with the graphics acceleration/hardware acceleration option in the browser, you even went for the surface diagnostic app, good on you for taking the first step.
It might be your web browser since it’s only affecting Netflix and youtube. If you have any add-ons to your browser disable all of them, then view a video in fullscreen, if it goes back to normal, enable your add-ons one by one and see find the add-on causing it.
If the add-on reset doesn’t work try uninstalling and reinstalling your web browser, a clean reinstall usually fixes most issues.
Another tip that works for some people on the forums is disabling “Fullscreen Optimizations”, right click on the icon for your web browser, click properties, then go to compatibility, in there find “Disable Fullscreen Optimizations”
If it’s outside the web browser it might be your video graphics drivers, open the start menu, search “Device manager” and open the first result, this will show you the list of devices on your computer, find “Display adapters” and under display adapters will be your graphics processing unit, right click on the name and click update drivers, after it finishes the process restart your computer.
Hope this helps,
JC