I haven't been able to reproduce this 100%, but I often hit it when attempting to type relatively quickly in any stylesheet.
Created attachment 381518 [details] Patch This seems to fix the issue no matter what I try, including similar actions to what caused the regression in the first place. I think this may be the fix! =D
Comment on attachment 381518 [details] Patch Clearing flags on attachment: 381518 Committed r251429: <https://trac.webkit.org/changeset/251429>
All reviewed patches have been landed. Closing bug.
<rdar://problem/56499702>