[New Multicolumn] Actual support for layout of column-span:all.

Source/core/rendering/RenderBlockFlow.cpp

Index: Source/core/rendering/RenderBlockFlow.cpp
diff --git a/Source/core/rendering/RenderBlockFlow.cpp b/Source/core/rendering/RenderBlockFlow.cpp
index 0513586dd5081be7a3de8f49d36c672de700222b..36285cff6b3532896a3cd2be4dc7f2a5c8c71fed 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -544,11 +544,44 @@ void RenderBlockFlow::setLogicalTopForChild(RenderBox* child, LayoutUnit logical
     }
 }
 
+class ColumnSpannerLayoutScope {
+public:
+    ColumnSpannerLayoutScope()
+        : m_spanner(0) { }
+
+    ~ColumnSpannerLayoutScope()
+    {
+        if (!m_spanner)
+            return;
+        m_spanner->flowThreadContainingBlock()->leaveColumnSpannerAfterLayout(m_spanner, m_spanner->containingBlock()->logicalHeight());
+    }
+
+    LayoutUnit enterSpanner(RenderBox* spanner, SubtreeLayoutScope& layoutScope)
+    {
+        ASSERT(!m_spanner);
+        m_spanner = spanner;
+        return m_spanner->flowThreadContainingBlock()->enterColumnSpannerBeforeLayout(m_spanner, m_spanner->containingBlock()->logicalHeight(), layoutScope);
+    }
+
+private:
+    RenderBox* m_spanner;
+};
+
 void RenderBlockFlow::layoutBlockChild(RenderBox* child, MarginInfo& marginInfo, LayoutUnit& previousFloatLogicalBottom)
 {
     LayoutUnit oldPosMarginBefore = maxPositiveMarginBefore();
     LayoutUnit oldNegMarginBefore = maxNegativeMarginBefore();
 
+    SubtreeLayoutScope layoutScope(*child);
+
+    ColumnSpannerLayoutScope columnSpannerLayoutScope;
+    if (child->isColumnSpanAll()) {
+        LayoutUnit adjustment = columnSpannerLayoutScope.enterSpanner(child, layoutScope);
+        // A spanner needs to start at an exact column boundary inside the flow thread, so that it
+        // doesn't bleed into a preceding column. That's what the adjustment is about.
+        setLogicalHeight(logicalHeight() + adjustment);
+    }
+
     // The child is a normal flow object. Compute the margins we will use for collapsing now.
     child->computeAndSetBlockDirectionMargins(this);
 
@@ -590,7 +623,6 @@ void RenderBlockFlow::layoutBlockChild(RenderBox* child, MarginInfo& marginInfo,
             previousFloatLogicalBottom = std::max(previousFloatLogicalBottom, oldLogicalTop + childRenderBlockFlow->lowestFloatLogicalBottom());
     }
 
-    SubtreeLayoutScope layoutScope(*child);
     if (!child->needsLayout())
         child->markForPaginationRelayoutIfNeeded(layoutScope);
 
@@ -1295,7 +1327,7 @@ LayoutUnit RenderBlockFlow::collapseMargins(RenderBox* child, MarginInfo& margin
     // If margins would pull us past the top of the next page, then we need to pull back and pretend like the margins
     // collapsed into the page edge.
     LayoutState* layoutState = view()->layoutState();
-    if (layoutState->isPaginated() && layoutState->pageLogicalHeight() && logicalTop > beforeCollapseLogicalTop) {
+    if (layoutState->isPaginated() && layoutState->pageLogicalHeight() && logicalTop > beforeCollapseLogicalTop && !child->isColumnSpanAll()) {

[Julien - ping for review, 2014/11/17 23:47:11]

It seems like we should just make column spanning items create new stacking
contexts (see example 27) so that it disable margin collapsing in some context.
This is done by updating RenderBlockFlow::createsNewFormattingContext().

[mstensho (USE GERRIT), 2014/11/18 13:29:26]

They already do create a new block formatting context (and I did update that
method), but doing that doesn't prevent margins of the BFC block from collapsing
with the surrounding margins. It only prevents margins of the BFC's children
from collapsing with BFC's own margins (and the BFC's surroundings).

What's example 27?

And "stacking context" was a typo, no?

[Julien - ping for review, 2014/11/19 17:54:34]

I missed that you actually changed the function in this patch. What I meant to
write (but missed the intent of the code) is that we should not get into the
collapsing margin logic if we shouldn't collapse them. I don't have a good
suggestion unfortunately as I don't know the collapsing margin code very well so
I could be missing something.
Sorry, I meant example 27 of the multi-column specification (scroll down a bit
from http://dev.w3.org/csswg/css-multicol/#column-span)
Yeah, I meant block formatting context.

[mstensho (USE GERRIT), 2014/11/21 15:39:24]

This code here is about the top margin. Being a BFC here or not doesn't really
matter. A new BFC also lives in a BFC established by some ancestor, and it will
collapse with its previous siblings and parents.

<div style="margin-bottom:100px;">xxx</div><div style="overflow:hidden;
margin-top:100px;"></div>

The margin between the two DIVs here will be 100px, even if the latter is a BFC.

But we digress: This isn't really about regular margin collapsing, but rather
about eating top margins of elements that end up as the first element in a
column. [1]

But hey, I just realized that this change isn't needed by any of the current
tests, so I'll just back it out for now. I will probably have to reintroduce
this later, since I have some more tests (and code) in the oven.

[1] http://dev.w3.org/csswg/css-break/#break-margins

         LayoutUnit oldLogicalTop = logicalTop;
         logicalTop = std::min(logicalTop, nextPageLogicalTop(beforeCollapseLogicalTop));
         setLogicalHeight(logicalHeight() + (logicalTop - oldLogicalTop));
@@ -1511,7 +1543,7 @@ LayoutUnit RenderBlockFlow::estimateLogicalTopPosition(RenderBox* child, const M
     // Adjust logicalTopEstimate down to the next page if the margins are so large that we don't fit on the current
     // page.
     LayoutState* layoutState = view()->layoutState();
-    if (layoutState->isPaginated() && layoutState->pageLogicalHeight() && logicalTopEstimate > logicalHeight())
+    if (layoutState->isPaginated() && layoutState->pageLogicalHeight() && logicalTopEstimate > logicalHeight() && !child->isColumnSpanAll())

[mstensho (USE GERRIT), 2014/11/21 15:39:24]

I'll remove this for now. No test covers it. See previous comment.

         logicalTopEstimate = std::min(logicalTopEstimate, nextPageLogicalTop(logicalHeight()));
 
     logicalTopEstimate += getClearDelta(child, logicalTopEstimate);