Inline split fixes#12699
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Fix fragment splitting algorithm to set FIRST/LAST_FRAGMENT_OF_ELEMENT flag correctly, account for border_padding for border_box and set zero border_padding between for new fragments.
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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#12699 (github issue number if applicable).
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- [ ] These changes do not require tests because _____
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