iPad PageList parsing using Javascript.

[Oh.Danny.Boy]

I understand this is not specifically a development forum, but I have a question regarding PageList parsing in an iOS environment. (I only see Development area for Kindle/Nook).

PageList and supporting targets are fairly straightforward. A list of anchors referring to their respective file (src), and a playOrder for well, ordering. My question is how do eReaders actually detect when anchors are in view?

For example, a document at 768px wide by 10,000px height may have 20 PageTargets (pg i, pg ii, pg 1, pg 2, pg 3 ... so on). What mechanism are ePub rendering engines on the web, desktop, and specifically iOS using to detect the closest anchor relative to a users position (XY coordinates)?

There seems to be nothing in Javascript or Objective-C which does this. Voodoo functionality.