Should we render documents in two passes to account for non-flow content

[BrianS]

Currently dompdf parses and renders a document in one pass. This results in some interesting but confusing functionality around, for example, fixed-position content. Normally one would expect fixed-position content to appear on all pages since it is fixed to the viewport and not part of the normal document flow. But dompdf renders fixed position content starting on the page currently being rendered. So if fixed-position content is located at the bottom of the document it will only render on the last page.

So the question is, do we want to modify how dompdf renders a document so that an inital pass is made to look for non-flow content? This would include fixed-position content as well as absolute-position content relative to the document body. The first pass of the document would pull out this content and render it starting with page 1, followed by a second pass to render all normally-flowed content and content positioned relative to other elements (e.g. absolute-position content inside relative-position content).

I have not read up on the W3 spec relating to this question, so any pointers would be helpful.