Text prompt automatically going to the end of the comment/post, even when editing

I'm not sure how to explain this so I'll start off with an example. Let's say i'm typing a comment that says "The cat ate the mouse". And now let's say that I wanted to add some detail, for example I wanted to specificy that the cat was big and black. I'd move the text prompt between "The" and "cat", and add in "big black". Expected result is "The big black cat ate the mouse". What happens sometimes however, is that the prompt automatically goes back to the end of the comment and part of what I wanted to add ("big black") gets written at the end of the message.
So for example the end result would be "The big bl cat ate the mouseack". Lots of my sentences make no sense when this happens since they get completely truncated, often in the middle of the words. The longer I'm editing, the higher chance there is of this happening. I usually go back a lot and this problem happens almost on every long post/comment I make (happened while typing this sentence for example). I've experienced this on firefox 54.0.1 and Chrome, OS is OS X 10.

I have no clear explanation of why this happens, but maybe it is linked to some kind of periodical process, such as auto-refreshing, or the saving of the draft that brings the prompt automatically to the end of the message.