I have the latest version of Xenforo 1.5.4, using Safari on a mac, if trying to copy text in a thread with the cursor - the screen "jumps."
This isn't true - at least, what's being described here is not typical.
Try it at at vB forum or any other editor window at any other software and you will see the same. No matter if TinyMCE or Redactor. The recent Safari does not work well with them.
You're right it does happen on vB as well.
Not only vB. It happens with every editor input field in every web interface. We experienced it when editing a newsletter in the editor of our newsletter service.
You have to switch browsers to prevent it. I agree it is very unfortunate. Hopefully Apple will provide an update soon.
I have been able to reproduce this issue using a fairly common rich text editor set up involving a contenteditable body inside an iframe. I've also been able to reproduce the error on several popular rich text editors including TinyMCE and CKEditor. As this issue is not exclusive to XenForo it suggests a browser bug.
I am in the process of creating a simplified reproduction case to send to Apple and I will update this thread if there are any updates on that. In the meantime, however, this is being marked as a browser issue.
Well, I wouldn't get your hopes up.
I opened a bug report in November 2014 relating to a specific RTL glitch and that was quickly closed as a duplicate. I think it was iOS 9.2 (way over a year later) that it was actually fixed. The annoying thing about duplicate reports though is you can't see other people's reports, and get no updates on reports you've made after they're closed. Still the same on iOS 9.3 beta... maybe iOS 10...