I faced this problem on one of my WordPress websites and a simple fix was not so simple, so I completely replaced the WYSIWYG editor that comes as default with the WordPress installation. I looked online and it seemed people were looking for solutions to actually fix the editing part of the script. But why bother?
I completely replaced it with this:
(This completely replaces your old WYSIWYG editor. So if you have any problems with the defualt one, this one is a perfect replacement for it!)
I tried everything to fix the issue I had previous to installing this, but really this WYSIWYG editor is much better than the default editor. Here is a screenshot of it:
When I mean I tried everything, I tried everything bar-reinstalling my entire WordPress installation otherwise that would have screwed up the many plugins I have installed and the many graphical and back-end changes I made to the WordPress installation. But as you can see and if you have indeed experienced WordPress you will see instantly that there are more options with this editor while also being more pleasant to look at. 😀 (Win-win!) It totally overwrites your previous WYSIWYG editor which is what makes this “FIX” much better than fixing the default back-end editor.
So this is a perfect solution to fix your problem with your WordPress WYSIWYG editor.