Reflow has happened by this point, so there's no need to null check GetSharedPageData()(). (There are loads of places that just use mPD unchecked in nsPageFrame on that basis.)

Sooo...obviously this is all a bit silly, undoing various steps that we carried out earlier. However, I tied myself up in knots a bit trying to refactor the code to avoid that. I'll come back to that and have another pass at it later, but for now everyone (including yourself, I'm sure) wants to just see a working implementation go out the door. The code changes are much more straightforward to follow this way, and lower risk which seems important if landing at code freeze time.

This way it's also pretty easy to put all changed code behind the pref, making it easy to turn off if things go wrong, which again seems desirable.

Let me know if I need to expand this comment a bit. It is perhaps not too obvious what I'm doing in the following lines of code, but maybe it is?