Gecko:PrintPreview

Some ideas on how to improve the print preview situation for Gecko 1.9.

Problems

Currently print preview is completely modal, because we can't allow the underlying document to change while we're print previewing it.
We don't allow any interaction with the print preview, such as changing margins or scaling.
The entire contents of the print preview window, including page shadows and any UI (well, there isn't any, but still...) are defined in Gecko print-related frame code. This is a big problem for anyone who might want to enhance print preview.
The print preview presentation doesn't necessarily have the same layout as the print presentation. In many ways it's still a screen presentation.

Fix the modality problem by cloning the document (and any subdocuments) and using the clone for printing/print preview.
- Replace plugins with an image snapshot of the current plugin state
- This is basically done.
Reimplement "print selection" and "print subset of frames" by modifying the clone before we print/print preview.
Do away with the print preview presentation. Instead, implement print preview by building the print presentation and then rendering snapshots of it to the screen.
- Make the print preview window a XUL chrome document containing a list of XUL <page> elements. These elements are leaf elements that simply render pages from some stored print presentation.
- There will be some API to determine (or possibly set) the properties of each page and how many pages are used by the print presentation; the chrome UI is responsible for creating/destroying page elements as needed.
- Chrome UI is responsible for rendering page shadows and all other window decorations.
Provide a way to reflow (or recreate frames for) the print presentation in response to changes in scale factor, margins, etc.