If I do another iteration (view the save-to-pdf generated PDF in Firefox, and print with our Save-to-PDF print target), then I get output that's differently (and more-thoroughly) broken in the macOS Preview app. Here's that PDF. macOS renders just the bottom-left quadrant of both pages here, scaled up to the full page size, it seems. (The other 3 quadrants of each page are pushed off the page & hence unreadable.)
Bug 1779202 Comment 14 Edit History
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If I do another iteration (view the save-to-pdf generated PDF in Firefox, and print with our Save-to-PDF print target), then I get output that's differently (and more-thoroughly) broken in the macOS Preview app. Here's that PDF. The macOS Preview app renders just the bottom-left quadrant of both pages here, scaled up to the full page size, it seems. (The other 3 quadrants of each page are pushed off the page & hence unreadable.)