The board software used is Fireboard. Unfortunately, it's a bug within the code.
I looked into it quite a lot and it's a conflict caused by other Javascript on the site.
Unfortunately, it's doesn't seem to be on Fireboard's priority list to fix.
I've made a few customisations to Fireboard, but playing with its Javascript libraries usually ends in disaster.
The best I could do at the moment is remove the button, but I haven't done that as I've been hoping for a fix.
You'll just have to go all in and post without previewing
You can always edit later!