The picture says it all...

Open the page of a project with a lengthy instructions/N&C value, created by someone other than you. (This doesn't happen on your own project pages because the actual input fields don't appear to have an issue.)
Firefox 66.0b3, Linux. Can't reproduce on Chromium.
/cc @thisandagain since this should be pretty high priority if it affects every Firefox user (needs repro on other devices / FF versions, but that is likely the case). I saw this happen on https://scratch.mit.edu (any project with long instructions/N&C).
Uh oh, this might be a problem I introduced with my recent change to that part of the site. I'll work on a fix right away.
Thanks for pointing this out, @towerofnix.
repro: firefox 77 on windows 10, same project
I repro'd it too, and made a much simplified use case using our html & css:
https://codepen.io/benjiwheeler/pen/earzje?editors=1100
Looks like the culprit may be this flexbox issue: https://stackoverflow.com/a/36247448
Sorry to burst in and if this isn't the right place to point this out, but apparently this also happens on Chrome? According to this post: https://scratch.mit.edu/discuss/topic/352810/ the user is using Chrome and encounters the same problem.
Update: nvm, they seem to be using Microsoft Edge
Most helpful comment
Uh oh, this might be a problem I introduced with my recent change to that part of the site. I'll work on a fix right away.
Thanks for pointing this out, @towerofnix.