LayoutUnit RenderFlexibleBox::preferredMainAxisContentExtentForChild(RenderBox* child) { Length flexBasis = flexBasisForChild(child); if (flexBasis.isAuto()) { LayoutUnit mainAxisExtent = hasOrthogonalFlow(child) ? child->logicalHeight() : child->maxPreferredLogicalWidth(); return mainAxisExtent - mainAxisBorderAndPaddingExtentForChild(child); } return std::max(LayoutUnit(0), computeMainAxisExtentForChild(child, MainOrPreferredSize, flexBasis)); } In the first return statement we need to clamp to 0 the same way we do in the second one. If mainAxisExtent < mainAxisBorderAndPaddingExtentForChild, then we are incorrectly returning a negative value.