See bug 959782 comment 0 for example. There are use cases where content authors want to adjust the scroll offset by a relative amount and not have it disrupt what the APZ is currently doing (with respect to scrolling or flinging). We could perhaps change the scrollBy implementation on the window (and DOM elements, if they have it) to play nicely with the APZ rather than behaving the same as scrollTo.

This bug is more of a implementation detail than a user-facing feature. There might be other ways to solve the problem that this bug is about, so I don't think this should block any releases. If there are specific apps (e.g. email) that would benefit from this bug being fixed, they should depend on this one and we can consider fixing it.

I think for e-mail we do indeed just want to fix the problem at the root by fixing bug 796474, which is roughly what :roc proposed on his recent blog post at http://robert.ocallahan.org/2014/02/implementing-virtual-widgets-on-web.html

This will be one use of the mechanism implemented in bug 1453425.

We now have a different scroll mechanism for scrollBy in bug 1453425 so I will close this.