Well, a bug report would have been nice then :)
Of course this one could have some advantages, I was just pointing out my bookmarklet that also provides that "collapse whole thread" feature.
That Chrome extension also has a neat "new postings" feature.
Also, your collapse/uncollapse button is placed on the right instead of left so its X position may vary, making it a bit more difficult to collapse comments in a series.
Good effort though.