Annotation anchoring in PDFs is inconsistent

[lenazun]

Steps to reproduce

In some PDFs, annotations are failing to anchor consistently. This means they are now showing as Orphans. See @judell 's exploration of one case here (points to an internal Hypothesis doc, because it contains private user information)
https://docs.google.com/document/d/1MFlsqnMkbWf9KZoppGKYO1-p6LRrruW2ixzJkF5rX-E/edit

Expected behaviour

Static PDFs should not have orphans, given that the anchoring should be consistent.

Actual behaviour

Anchoring is inconsistent. Sometimes annotations anchor and sometimes they fail to anchor, and end up in the Orphans tab.

[lenazun]

Part of the solution proposed is layed out here
https://trello.com/c/hMV9J8ri/454-revisit-timeouts-for-reanchoring

[lenazun]

Revisit timeouts for reanchoring

[judell]

OK I think I've found a reproducible correlation, but things have been so wonky I'd appreciate confirmation.

http://jonudell.net/h/orphan-misanchoring-correlates-with-pdf-scroll-location.mp4

The setup:

URL: http://jonudell.net/h/power-of-habit.pdf
Group: https://hypothes.is/groups/1LxDv3K9/test-group

The TL;DR I can reliably mis-anchor if I'm on or near the PDF page where the annotations live, and I can reliably anchor if I'm far way in the PDF from those (or, in this case, any) annotations.

Anchoring timeout is /not/ a factor here, as I've bumped it up from 500 to 500000.

If this correlation holds, it suggests that we may need to look into how we interact with what I believe to be a lazy-rendering strategy on the part of PDF.js.

[judell]

I'm closing this in favor of a new issue I'll create in the client repo.