I often switch between a high-resolution external screen and my built-in laptop screen. I've noticed that when I start on the high-res external screen and then switch to the low-res laptop screen, the cells in any open notebooks get cut-off. It goes from this:

To this:

On reloading of the page or resizing of the window, the cells go back to normal. It's fairly easy to work-around, but I thought I'd let you know about the issue!
Thanks, Jake! This might be a browser issue, avoiding triggering redraws when circumstances change when it should repaint the whole page. I'm not sure we can detect this if the browser is hiding it from us, but we can investigate if there's an event coming that we are somehow suppressing.
Confirming happened to me today.
Fixing the bug locally was simple:
by zooming out to 90% and zooming back in to 100% with Chrome Ctrl + - / +
Experiencing the same issue with Firefox 67.0.4 on macOS 10.14.5 after disconnecting from 1920x1080 external monitors

Most helpful comment
Confirming happened to me today.
Fixing the bug locally was simple:
by zooming out to 90% and zooming back in to 100% with Chrome
Ctrl+-/+