Created attachment 8742506 [details] testcase 1 STR: 1. Load attached testcase. EXPECTED RESULTS: Nothing should escape from the black bordered area. ACTUAL RESULTS: The "aaaa...." and its pink background runs off the right side of the black bordered area. This happens as a result of honoring modern flexbox's "min-width:auto" behavior. (Note that the spec's special case for "overflow: [non-visible]" doesn't apply here, because that special case is about "overflow" being set on a *flex item* -- and in this testcase, "overflow" is set on the *child* of a flex item.) We may need to consider suppressing "min-width:auto" behavior for flex items in a -webkit-box container.
Once webkit prefix support is enabled, this bug is responsible for the header toolbar at http://m.hupu.com/bbs/1048 overflowing, as pointed out in https://github.com/webcompat/web-bugs/issues/2450#issuecomment-211212980 and my response to that comment. The element with class "board-con" there ends up too wide in Firefox Nightly, because the default "min-width:auto" behavior makes it refuse to be smaller than its kids' min-content width (which happens to be huge). (On the plus side, though, we behave similarly when webkit prefix support is disabled -- at least for that site. So this is not a regression for us -- so at this point, this doesn't need to block bug 1259345.)
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