[css-grid] Implementation of Baseline Self-Alignment

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

Index: third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp b/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
index 605c990b65b93b50879aac549a6e53232b9cc04a..c82869d7fdd1d998a05f61f7a86f83e4995f91c8 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
@@ -2489,8 +2489,15 @@ int LayoutBlockFlow::lineCount(const RootInlineBox* stopRootInlineBox) const {
   return count;
 }
 
+bool LayoutBlockFlow::isDescentBaseline() const {
+  if (isOrthogonalWritingModeRoot())
+    return !styleRef().isHorizontalWritingMode();

[kojii, 2016/11/16 17:18:06]

Just curious, since I don't understand, why do we need this? I thought just
isFlippedLinesWritingMode() does the work, but if not, having a comment would
help.

[jfernandez, 2016/11/16 18:50:07]

In the general case, we don't compute baselines for orthogonal boxes; that's why
the fitstLineBoxBaseline returns -1 when isWritingModeRoot(). However, we
allow that for grid items, since we can use baseline alignment for orthogonal
grid items along the grid's column axis.

When the grid container is horizontal, then both vertical-lr and vertical-rl
will
be placed at the left edge. In case of a vertical grid container, grid items
will
be horizontal, hence we should use always the font ascent to determine
the baseline.

Perhaps a comment in the code could make this clearer. However, I still have
the feeling that perhaps this logic should be better implemented in the
specific layout models. So far, I can't do it because of what you commented
about using always logicalTop(). 

In any case, once I get a valid patch I'll rethink the whole issue to see
whether I can find a more general and solution. 

[kojii, 2016/11/18 04:14:53]

Oh I think I understand this. I think the missing piece is that this function
lacks an argument to compute "first baseline for whom", and you assume it is for
its parent, so the baseline is -1 if orthogonal root. I was thinking about using
this function for non-parents, so the logic is fine.
Sounds great, thank you for working on this again.

+  return parent() ? parent()->styleRef().isFlippedLinesWritingMode()
+                  : styleRef().isFlippedLinesWritingMode();
+}
+
 int LayoutBlockFlow::firstLineBoxBaseline() const {
-  if (isWritingModeRoot() && !isRubyRun())
+  if (isWritingModeRoot() && !isRubyRun() && !isGridItem())
     return -1;
   if (!childrenInline())
     return LayoutBlock::firstLineBoxBaseline();
@@ -2500,7 +2507,11 @@ int LayoutBlockFlow::firstLineBoxBaseline() const {
     if (!fontData)
       return -1;
     return (firstLineBox()->logicalTop() +
-            fontData->getFontMetrics().ascent(firstRootBox()->baselineType()))
+            (!isDescentBaseline()
+                 ? fontData->getFontMetrics().ascent(
+                       firstRootBox()->baselineType())
+                 : fontData->getFontMetrics().descent(
+                       firstRootBox()->baselineType())))
         .toInt();
   }
   return -1;