As noted in this comment: https://phabricator.services.mozilla.com/D53709?id=194022#inline-324289 We have some DEUBG code that requires DesiredSize to be assigned to a value: https://searchfox.org/mozilla-central/rev/131338e5017bc0283d86fb73844407b9a2155c98/layout/generic/nsBlockReflowContext.cpp#286-287,291,304-305 IIRC, there's also some other block reflow code that actually used the size even when BreakBefore was returned, which is bogus. (The attached WIP avoided that code, but I've forgot the exact details.)
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As noted in this comment: https://phabricator.services.mozilla.com/D53709?id=194022#inline-324289 We have some DEBUG code that requires DesiredSize to be assigned to a value: https://searchfox.org/mozilla-central/rev/131338e5017bc0283d86fb73844407b9a2155c98/layout/generic/nsBlockReflowContext.cpp#286-287,291,304-305 IIRC, there's also some other block reflow code that actually used the size even when BreakBefore was returned, which is bogus. (The attached WIP avoided that code, but I've forgot the exact details.)