Here's a modified testcase, for comparison, where I've changed things to use some padding instead of some border on the outermost div. We get things correct in this case. So this demonstrates that this is specifically an issue with us mis-handling the border and only the border (either double-counting it, or forgetting to subtract it away, or something like that) on the abspos containing block -- it's not a problem for the padding.
Bug 1765668 Comment 4 Edit History
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Here's a modified testcase, for comparison, where I've changed things to use some padding instead of some border on the outermost div. We get things correct in this case. So this demonstrates that this is specifically an issue with us mis-handling the border and only the border (either double-counting it, or forgetting to subtract it away, or something like that) on the abspos containing block, when we compute the hypothetical / static position of the abspos thing, and it's not a problem for the padding.