Here's a much more reduced testcase. The issue here is that Firefox and Safari implement an older version of the flexbox intrinsic-sizing algorithm, which doesn't take the 400px `flex-basis` into account; so we end up with a container size that's too small, as shown in both lines in this testcase. (And then we *do* take that flex-basis into account when doing layout inside of the intrinsic size, which leads to stuff getting squished and overlapping, given the testcase's explcitly-specified `min-width:0` and default nonzero `flex-shrink` on the first item, shown on the second line in this testcase.)
Bug 1996140 Comment 5 Edit History
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Here's a much more reduced testcase. The issue here is that Firefox and Safari implement an older version of the flexbox intrinsic-sizing algorithm, which doesn't take the 400px `flex-basis` into account; so we end up with a container size that's too small, as shown in both lines in this testcase. (And then we *do* take that flex-basis into account when doing layout inside of the intrinsic size, which leads to stuff getting squished and overlapping, given the website's explicitly-specified `min-width:0` and default nonzero `flex-shrink` on the first item, shown on the second line in this testcase.)