Performance issue when click the view source button in the WYSIWYG editor

I moved the diff computation in a Web Worker, and I'm showing a notification message (if the diff computation takes more than 1s) that can be used to cancel the selection restore. The notification message also shows a progress bar, but the progress is currently fake. I had to update the realtime editing code as well because the same code is used to restore the selection after a remote change / patch is applied in a realtime editing session.