Fix absolute-flow's auto positioning
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If an absolute positioned flow has no top, bottom, left, right property, its hypothetical box position should be the margin-end of its previous sibling, not the border-end.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#12676 (github issue number if applicable).
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- [x] There are tests for these changes
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