Maybe I’ve been browsing 9gag waaaay too much, but I often miss a scroll to top link, when reading long threads, seeing as I usually scroll to top manually so I can click on the “Show unread posts since epoch.” link.
A good idea, right? Right! 
Cheers!
Isn’t that what the “Home” button on your keyboard is for? 
Mike
Maybe, but seeing as the only time I use the keyboard, while browsing the web, is when I have to type stuff, then it’s annoyance that I have to let go of the mouse, to press home, and then grab the mouse again. Besides, there’s no home key on my laptop, only some obscure key combination that acts as a home key, so it’s even more annoying on my laptop.
I press back.
You have two hands dude!
Yes, but the backspace on the wrong side of the keyboard. It’s all about comfort, you know! xD
<div style="position:absolute;right:5px;bottom:0px;border:1px solid black;background-color:white;">
<a href="#" title="to top">to top</a>
</div>
Is all it takes, more or less. 
That would intersect the sidebar.
s/right:5px;/left:5px;/
That would intersect the usernames and avatars.
Well, yes. But it’ll only cover it in posts where you can’t see the content yet. Wouldn’t be a big loss, imo.
Use a rocker gesture.