Videos
The problem might be related to a Google Chrome extension.
Extensions are synced across computers through the Google Accounts settings. Some extensions save the extension settings on the cloud, so they are synced across computers, too.
The above is because the keyboard shortcuts mentioned in your answer, Ctrl + Shift + 1 and Ctrl + Shift + 2, should be listed in the help articles about keyboard shortcuts for Chrome and YouTube.
To view the YouTube keyboard shortcuts, go to https://youtube.com and press Shift + ?
To view the Chrome keyboard shortcuts, go to the help article Chrome keyboard shortcuts.
Next time, you can just reset Chrome instead of reinstalling Chrome when troubleshooting a problem like this. For details, see Reset Chrome settings to default. Still, before doing this, you might want to try using Chrome in incognito mode with all the extensions disabled.
Ok -- I finally figured this out after a bit of experimmentation.
Make sure the Chrome browser has focus (click somewhere in the browser window), then press Ctrl+Shift+1 (that's the number one), or Ctrl+Shift+2. That will cause Chrome to zoom in and out in a weird way that doesn't show up on the Zoom control in the three-dots menu in the upper-right corner. It will still show 100% Zoom, even though you're zoomed in (or out).
I have no idea what purpose this serves. And it's tied to your Google account somehow, so it affects all your Chrome browsers on all your devices. It will zoom in and out of ANY web-page, not just YouTube, but it has weird effects on YouTube that mess up video playback as I described.
Anyway, if your YouTube player in your Chrome browser is zoomed-in in a weird way, or ANY web=page you frequent is not fitting right, try Ctrl+Shift+1 and Ctrl+Shift+2 to zoom in and out to make it fit properly again.