This is a hack: we're making the textarea have the same size as CKEditor
so when it's replaced the page won't jump.
A very similar hack was removed in commit e844b0b2. Back then I thought
this was a small issue we could live with, but the user experience turns
out to be a bit annoying, and it makes tests fail sometimes because
Capybara is trying to click something when the page jumps, and so it
misses the click.
4ab3498892