Right now our mixed page size is restricted to only switching the width/height of pages output in save-to-PDF. That's because, as noted in a code comment in our cocoa code, the Apple API did not seem to support setting page size properly.

However, I've found that you can create a PMPaper with a custom size, use that to create a PMPageFormat, and pass that to PMSessionBeginPageNoDialog.

I haven't tested this yet, but if it works we should be able to fully support CSS page size in the pages that we output during save-to-PDF.

Assuming that works, I think the rest of the changes required will be limited to PrintedSheetFrame::ComputeSheetSize.

Assuming that works, I think the rest of the changes required will be limited to PrintedSheetFrame::ComputeSheetSize.