<rdar://problem/98673438>

This is about integral based alignment vs. non-integral based box model values e.g: vertical-align: -0.5px; -> turns into 0px margin-bottom: 0.5px; -> stays at 0.5px these tests trigger this mismatch through the combination of 'font-size: 15px' and 'align/margin -> 0.5em'. Legacy line layout is historically integral based, while IFC is not, currently it is forced to match legacy behavior through explicit flooring/ceiling of certain values.

This is actually broken (and probably has been broken forever) with legacy line layout and now IFC just matches this broken behavior. :(

Not forcing integral rounding fixes the issue (see below). So while 252825@main looks like the regression point, it never worked in IFC either, it was just covered by the directional rounding bug -which got corrected in 252825@main. The actual cause of this bug is the explicit rounding we need to do in IFC to match legacy line layout behavior. diff --git a/Source/WebCore/layout/formattingContexts/inline/InlineLevelBox.h b/Source/WebCore/layout/formattingContexts/inline/InlineLevelBox.h index 0ac8876b9a15..16420dd6b53c 100644 --- a/Source/WebCore/layout/formattingContexts/inline/InlineLevelBox.h +++ b/Source/WebCore/layout/formattingContexts/inline/InlineLevelBox.h @@ -123,11 +123,11 @@ private: // FIXME: Remove legacy rounding. void setLogicalWidth(InlineLayoutUnit logicalWidth) { m_logicalRect.setWidth(logicalWidth); } - void setLogicalHeight(InlineLayoutUnit logicalHeight) { m_logicalRect.setHeight(roundToInt(logicalHeight)); } - void setLogicalTop(InlineLayoutUnit logicalTop) { m_logicalRect.setTop(logicalTop >= 0 ? roundToInt(logicalTop) : -roundToInt(-logicalTop)); } - void setAscent(InlineLayoutUnit ascent) { m_ascent = roundToInt(ascent); } - void setDescent(InlineLayoutUnit descent) { m_descent = roundToInt(descent); } - void setLayoutBounds(const LayoutBounds& layoutBounds) { m_layoutBounds = { InlineLayoutUnit(roundToInt(layoutBounds.ascent)), InlineLayoutUnit(roundToInt(layoutBounds.descent)) }; } + void setLogicalHeight(InlineLayoutUnit logicalHeight) { m_logicalRect.setHeight(logicalHeight); } + void setLogicalTop(InlineLayoutUnit logicalTop) { m_logicalRect.setTop(logicalTop); } + void setAscent(InlineLayoutUnit ascent) { m_ascent = ascent; } + void setDescent(InlineLayoutUnit descent) { m_descent = descent; } + void setLayoutBounds(const LayoutBounds& layoutBounds) { m_layoutBounds = { InlineLayoutUnit(layoutBounds.ascent), InlineLayoutUnit(layoutBounds.descent) }; } void setIsFirstBox() { m_isFirstWithinLayoutBox = true; } void setIsLastBox() { m_isLastWithinLayoutBox = true; }