When scrolling large pages, it is nice to be able to view a single thread and not loose your place on the parent page.
Good point.
When you navigate back, the browser restores the scroll position on the page you left, so this would also be the UX with full page reloads.
Restoring the scroll position doesn’t always work when JavaScript is used to render a page. I am not a fan of single page apps because they often break these subtle features.
Scroll position is maintained (and need not be restored) if the thread opens in a modal.
