[LayoutNG] Set the correct TextAffinity for PositionForPoint0

This patch implements the behavior that, when clicking at the end of
a line, we set an upstream caret selection to indicate that the
the selection is at the end of the first line instead of the beginning
of the second line, though they may have the same DOM position.

This patch fixes accessibility/selection-affinity.html, whose failure
is no longer relevant to Range.getClientRects().

Bug: 811502
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0e2d9a9b93e9371a0fb604a0550a376b6035e09d
Reviewed-on: https://chromium-review.googlesource.com/1111466
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569550}

third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-features=LayoutNG

 # These tests currently fail when they run with --enable-blink-features=LayoutNG
 # See https://crbug.com/591099.
 
-# Need support for Range.getClientRects() and Range.getBoundingClientRect()
-crbug.com/755750 accessibility/selection-affinity.html [ Failure ]
-
 # Non-interoperable behavior not worth to fix.
 crbug.com/591099 fast/text/apply-start-width-after-skipped-text.html [ Skip ]
 

third_party/blink/renderer/core/layout/ng/inline/ng_caret_position.cc

@@ -289,25 +289,40 @@ NGCaretPosition ComputeNGCaretPosition(const PositionWithAffinity& position) {
 }
 
 Position NGCaretPosition::ToPositionInDOMTree() const {
+  return ToPositionInDOMTreeWithAffinity().GetPosition();
+}
+
+PositionWithAffinity NGCaretPosition::ToPositionInDOMTreeWithAffinity() const {
   if (!fragment)
-    return Position();
+    return PositionWithAffinity();
   switch (position_type) {
     case NGCaretPositionType::kBeforeBox:
       if (!fragment->GetNode())
-        return Position();
-      return Position::BeforeNode(*fragment->GetNode());
+        return PositionWithAffinity();
+      return PositionWithAffinity(Position::BeforeNode(*fragment->GetNode()),
+                                  TextAffinity::kDownstream);
     case NGCaretPositionType::kAfterBox:
       if (!fragment->GetNode())
-        return Position();
-      return Position::AfterNode(*fragment->GetNode());
+        return PositionWithAffinity();
+      return PositionWithAffinity(Position::AfterNode(*fragment->GetNode()),
+                                  TextAffinity::kUpstreamIfPossible);
     case NGCaretPositionType::kAtTextOffset:
       DCHECK(text_offset.has_value());
       const NGOffsetMapping* mapping =
           NGOffsetMapping::GetFor(fragment->GetLayoutObject());
-      return mapping->GetFirstPosition(*text_offset);
+      const Position position = mapping->GetFirstPosition(*text_offset);
+      if (position.IsNull())
+        return PositionWithAffinity();
+      const NGPhysicalTextFragment& text_fragment =
+          ToNGPhysicalTextFragment(fragment->PhysicalFragment());
+      const TextAffinity affinity =
+          text_offset.value() == text_fragment.EndOffset()
+              ? TextAffinity::kUpstreamIfPossible
+              : TextAffinity::kDownstream;
+      return PositionWithAffinity(position, affinity);
   }
   NOTREACHED();
-  return Position();
+  return PositionWithAffinity();
 }
 
 }  // namespace blink

third_party/blink/renderer/core/layout/ng/inline/ng_caret_position.h

@@ -32,6 +32,7 @@ struct NGCaretPosition {
   bool IsNull() const { return !fragment; }
 
   Position ToPositionInDOMTree() const;
+  PositionWithAffinity ToPositionInDOMTreeWithAffinity() const;
 
   const NGPaintFragment* fragment = nullptr;  // owned by root LayoutNGMixin
   NGCaretPositionType position_type;

third_party/blink/renderer/core/paint/ng/ng_paint_fragment.cc

@@ -439,16 +439,12 @@ PositionWithAffinity NGPaintFragment::PositionForPointInText(
       text_offset < text_fragment.EndOffset()) {
     const Position position = NGOffsetMapping::GetFor(GetLayoutObject())
                                   ->GetFirstPosition(text_offset);
-    // TODO(xiaochengh): Adjust TextAffinity.
     return PositionWithAffinity(position, TextAffinity::kDownstream);
   }
   const NGCaretPosition unadjusted_position{
       this, NGCaretPositionType::kAtTextOffset, text_offset};
-  const Position adjusted_position =
-      BidiAdjustment::AdjustForHitTest(unadjusted_position)
-          .ToPositionInDOMTree();
-  // TODO(xiaochengh): Adjust TextAffinity.
-  return PositionWithAffinity(adjusted_position, TextAffinity::kDownstream);
+  return BidiAdjustment::AdjustForHitTest(unadjusted_position)
+      .ToPositionInDOMTreeWithAffinity();
 }
 
 PositionWithAffinity NGPaintFragment::PositionForPointInInlineLevelBox(