Auto merge of #20868 - tigercosmos:gg1, r=emilio

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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