use TextPointer in another thread

I have this project dealing with RichTextBox.

Current issue I am having is with the response of the algorithm reformatting the RichTextBox contents in real time when user types.

It's slower than I expected so I decided to separate out the algorithm in another thread and update the UI asynchronously.

The problem is now, When I run the algorithm in the background worker, I am keep getting CLR error code 0x80131623, on new spawned thread, which must be due to accessing Textpointer or Run part of the richtextbox.

Deep copy would no use since I need the saved pointer information of the original richtextbox UI. I was thinking if shallow copy would fix this issue, but I wasn't sure..