fix logic in overflow_direction, also add a FIXME
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This PR fix#19477, which the logic is obviously wrong.
However, it is impossible to add a test for this.
Due to #20867, the ` overflow_direction` is a hard code now. It never go to other conditions.
I add a FIXME there, and also add a SPEC link.
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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 #19477(github issue number if applicable).
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- [ ] There are tests for these changes OR
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20868)
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