I'm starting to worry about sounding condescending and I don't mean to. I've been in the industry since its inception. Literally.
I've turned all of my other extensions off with nothing active aside from the Malwarebytes extension. It does it every time unless I turn it on. Again, not meaning to sound condescending, but you do understand that no extension or browser activity can intercept (obviously the Malwarebytes one is, however.) Shift+a character or symbol? Those are typing keystrokes by definition. I'm just trying to figure out why Malwarebytes apparently is trapping this sequence and why the result is what I posted above in FB.
Whatever happens is particularly odd because when you attempt to leave a page you have commented on and not submitted, FB always pops a dialog saying your post will be lost, however; when I hit Shift+s during a comment, the blank above just happens. No dialog, unless I attempt to refresh the page or go <back.> Either of those actions produces the warning dialog.
Would you be so kind as to try it and see if you experience the same result? That would help. It certainly could just be unique to my configurations. A first step, as I asked in the OP, is to see if it is a duplicable result for at least one other person.
It's been happening since the beta. I just never got frustrated enough to tie it all together until today when I had to make a post with the name Susan in it and couldn't even use her name in the post. It probably didn't happen until a few days after the installation of the beta version because I probably don't use capital S's that much so I just never tied the two together. I just thought it was an anomaly with FB to begin with. FB has its own share of quirks in browsers also, and there's no use in reporting anything to them because they are about as supportive as <insert name of your least favorite company for customer support here> is. At the end of the day, it ended up not being them anyway.