I noticed that the 1px borders in testcase 1 don't show up for me in print-preview (due to them being scaled-down and then rounded to zero), which makes it slightly-harder to visualize. I imagine they can be seen with Retina/HiDPI resolutions, but they're invisible on my non-HiDPI monitor. Anyway: here's a testcase with 3px borders instead, to make them more-likely to show up in print preview, for easier analysis/visualization.)
Bug 1772396 Comment 2 Edit History
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I noticed that the 1px borders in testcase 1 don't show up for me in print-preview (due to them being scaled-down for the small print-preview visualization, which effectively rounds them to zero). This makes it slightly-harder to visualize. I imagine they can be seen with Retina/HiDPI resolutions, but they're invisible on my non-HiDPI monitor. Anyway: here's a testcase with 3px borders instead, to make them more-likely to show up in print preview, for easier analysis/visualization.)