A line that doesn't fit in a column shouldn't pull preceding floats to the next column.

Source/core/layout/LayoutBlockFlow.cpp

Index: Source/core/layout/LayoutBlockFlow.cpp
diff --git a/Source/core/layout/LayoutBlockFlow.cpp b/Source/core/layout/LayoutBlockFlow.cpp
index 2318e6c9ea19f7e7a9a3e50e027107ba06936955..cb5fd36327d94a14d684da0d80ef435084120eab 100644
--- a/Source/core/layout/LayoutBlockFlow.cpp
+++ b/Source/core/layout/LayoutBlockFlow.cpp
@@ -774,15 +774,42 @@ void LayoutBlockFlow::adjustLinePositionForPagination(RootInlineBox& lineBox, La
 
     int lineIndex = lineCount(&lineBox);
     if (remainingLogicalHeight < lineHeight || (shouldBreakAtLineToAvoidWidow() && lineBreakToAvoidWidow() == lineIndex)) {
+        // We need to insert a break now, either because there's no room for the line in the
+        // current column / page, or because we have determined that we need a break to satisfy
+        // widow requirements.
         if (shouldBreakAtLineToAvoidWidow() && lineBreakToAvoidWidow() == lineIndex) {
             clearShouldBreakAtLineToAvoidWidow();
             setDidBreakAtLineToAvoidWidow();
         }
-        LayoutUnit totalLogicalHeight = lineHeight + std::max<LayoutUnit>(0, logicalOffset);
-        LayoutUnit pageLogicalHeightAtNewOffset = pageLogicalHeightForOffset(logicalOffset + remainingLogicalHeight);
         setPageBreak(logicalOffset, lineHeight - remainingLogicalHeight);
-        if (((lineBox == firstRootBox() && totalLogicalHeight < pageLogicalHeightAtNewOffset) || (!style()->hasAutoOrphans() && style()->orphans() >= lineIndex))
-            && !isOutOfFlowPositioned() && !isTableCell()) {
+        // Decide whether to insert the break before the current line, or before the entire block.
+        bool wantsStrutOnBlock = false;
+        if (!style()->hasAutoOrphans() && style()->orphans() >= lineIndex) {

[leviw_travelin_and_unemployed, 2015/08/10 19:46:19]

It may be worth extracting this if/else block (determining if we want the strut
on the block) into a named function for ease of reading.

[mstensho (USE GERRIT), 2015/08/10 20:14:45]

Done.

That helper function got some papayawhip-colored parameter soup, but I still
like it better than what I had before.

+            // Not enough orphans here. Push the entire block to the next column / page as an
+            // attempt to better satisfy the orphans requirement.
+            wantsStrutOnBlock = true;
+        } else if (lineBox == firstRootBox() && logicalOffset == borderAndPaddingBefore()) {
+            // This is the first line in the block. We can take the whole block with us to the next
+            // page or column, rather than keeping a content-less portion of it in the previous
+            // one. Only do this if the line is flush with the content edge of the block,
+            // though. If it isn't, it means that the line was pushed downwards by preceding floats
+            // that didn't fit beside the line, and we don't want to move all that, since it has
+            // already been established that it fits nicely where it is.
+            LayoutUnit totalLogicalHeight = lineHeight + std::max<LayoutUnit>(0, logicalOffset);
+            LayoutUnit pageLogicalHeightAtNewOffset = pageLogicalHeightForOffset(logicalOffset + remainingLogicalHeight);
+            // It's rather pointless to break before the block if the current line isn't going to
+            // fit in the same column or page, so check that as well.
+            if (totalLogicalHeight < pageLogicalHeightAtNewOffset)
+                wantsStrutOnBlock = true;
+        }
+        // If we want to break before the block, one final check is needed, since some block object
+        // types cannot handle struts.
+        if (wantsStrutOnBlock && !isOutOfFlowPositioned() && !isTableCell()) {
+            // Note that when setting the strut on a block, it may be propagated to parent blocks
+            // later on, if a block's logical top is flush with that of its parent. We don't want
+            // content-less portions (struts) at the beginning of a block before a break, if it can
+            // be avoided. After all, that's the reason for setting struts on blocks and not lines
+            // in the first place.
             setPaginationStrut(remainingLogicalHeight + std::max<LayoutUnit>(0, logicalOffset));
         } else {
             delta += remainingLogicalHeight;