Sorry about this, though I haven't seen the problem and have just tried to replicate it unsuccessfully. I hope the webmaster has some thoughts on this.
FYI, I too have problems pasting in to the main editing window. Once I've pasted some text, sometimes it won't let me navigate properly with the arrow keys, as if the pasted text is not there, or only the pasted text is there (previously typed text are not able to be navigated to with the arrow keys).
Sorry about this, though I haven't seen the problem and have just tried to replicate it unsuccessfully. I hope the webmaster has some thoughts on this.
It doesn't look like an issue of this particular website. It is just an input, I don't see how this can be the fault of the script (website platform), if there are any problems then it must be your browser's fault. Have you tried using another browser?