Prioritize first-lines over orphans when deciding whether to propagate a strut.

third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 780 matching lines @@
 
     // Similar to how we apply clearance. Go ahead and boost height() to be the place where we're going to position the child.
     setLogicalHeight(logicalHeight() + (newLogicalTop - logicalTop));
 
     // Return the final adjusted logical top.
     return newLogicalTop;
 }
 
 static bool shouldSetStrutOnBlock(const LayoutBlockFlow& block, const RootInlineBox& lineBox, LayoutUnit lineLogicalOffset, int lineIndex, LayoutUnit pageLogicalHeight)
 {
-    bool wantsStrutOnBlock = false;
-    if (!block.style()->hasAutoOrphans() && block.style()->orphans() >= lineIndex) {
-        // 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 == block.firstRootBox() && lineLogicalOffset == block.borderAndPaddingBefore()) {
+    if (lineBox == block.firstRootBox()) {
         // 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.
+        // fits nicely where it is. In this case we have a class "C" break point [1] in front of
+        // this line.
+        //
+        // [1] https://drafts.csswg.org/css-break/#possible-breaks
+        if (lineLogicalOffset > block.borderAndPaddingBefore())
+            return false;
+
         LayoutUnit lineHeight = lineBox.lineBottomWithLeading() - lineBox.lineTopWithLeading();
         LayoutUnit totalLogicalHeight = lineHeight + lineLogicalOffset.clampNegativeToZero();
         // 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 <= pageLogicalHeight)
-            wantsStrutOnBlock = true;
+        if (totalLogicalHeight > pageLogicalHeight)
+            return false;
+    } else {
+        if (block.style()->hasAutoOrphans() || lineIndex > block.style()->orphans())
+            return false;
+
+        // Not enough orphans here. Push the entire block to the next column / page as an attempt to
+        // better satisfy the orphans requirement.
+        //
+        // Note that we should ideally check if the first line in the block is flush with the
+        // content edge of the block here, because if it isn't, we should break at the class "C"
+        // break point in front of the first line, rather than before the entire block.
     }
-    return wantsStrutOnBlock && block.allowsPaginationStrut();
+    return block.allowsPaginationStrut();
 }
 
 void LayoutBlockFlow::adjustLinePositionForPagination(RootInlineBox& lineBox, LayoutUnit& delta)
 {
     // TODO(mstensho): Pay attention to line overflow. It should be painted in the same column as
     // the rest of the line, possibly overflowing the column. We currently only allow overflow above
     // the first column. We clip at all other column boundaries, and that's how it has to be for
     // now. The paint we have to do when a column has overflow has to be special. We need to exclude
     // content that paints in a previous column (and content that paints in the following column).
     //
@@ skipping 2198 matching lines @@
     // FIXME: Glyph overflow will get lost in this case, but not really a big deal.
     GlyphOverflowAndFallbackFontsMap textBoxDataMap;
     for (ListHashSet<RootInlineBox*>::const_iterator it = lineBoxes.begin(); it != lineBoxes.end(); ++it) {
         RootInlineBox* box = *it;
         box->computeOverflow(box->lineTop(), box->lineBottom(), textBoxDataMap);
     }
     return childrenOverflowChanged;
 }
 
 } // namespace blink