+++ This bug was initially created as a clone of Bug #1498812 +++ As a quick fix, bug 1498812 is simply going to switch the session store to recording the visual viewport offset in lieu of the scroll position of the layout viewport. While this is better than the current situation, it's still not perfect because for cases where the two viewport positions differ, we won't be correctly restoring the position of the layout viewport and therefore any fixed-position page elements attached to the layout viewport won't appear in their previous position. Therefore, when the two viewport positions differ, we need to record and restore both of them. This affects both the current scroll positions kept by the session store, as well as the scroll positions stored in session history (in the PresState and the SHEntry itself).
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