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I can reproduce the flicker/crash with the post in the video (https://xenforo.com/community/threads/independent-transformable-invisible-links.83634/#post-830910). When I initially highlight, it highlights the whole thing (which isn't necessarily correct). When I try to change the selection at the top or bottom, it triggers the issue. Amusingly, clicking away to stop it sometimes causes the browser to scroll to the top of the page on its own (and then crash).

The bad news is that I reproduced it as a guest. The natural expectation would be for this to be related to the selection quoting stuff, but that code doesn't load for a guest.