I have a similar problem but found this thread first so am not creating a new thread, since the answer would be the same:
Moonchild wrote: ↑
we can do to even analyse
it, let alone guess at how to satisfy their scripting so it works as it should.
For me I usually can't type in the comment field at all. If I type it elsewhere and paste it in, that often works but not always. If I reply to a comment so that it automatically prepends the username, then I will be able to type in the comment field itself. BUT, if I hit backspace it will automatically move the cursor to just before the tagged name of the person I'm replying to, so it is very easy to delete that instead of what I actually intended to delete.
The obvious best workaround is to stop using Facebook. I was doing really well with that for a good six weeks or so, but have slowly started back into it *sigh*
I've not experienced this issue on any other website.