@Magnus Thanks for checking it out. I tried FUTO, and somehow I managed to reproduce it randomly. It looks like the keyboard’s auto-suggestion which displays an underline to the word at the cursor randomly breaks the CM’s state or contenteditable state of the webview.
Inkdrop does nothing when moving the cursor.
It looks like FUTO is still in beta, and I think this issue would be theirs rather than CodeMirror’s.
So, I can’t fix this issue. Maybe you can report it to FUTO’s developers.
It looks strange, but I can’t reproduce it on my account.
Does it happen every time?