[LayoutNG] Handle upstream-after-line-break input in caret resolution

When the input is upstream after a line break, LayoutNG caret resolution
algorithm should resolve at the downstream position instead. This patch
does that and fixes 4 crashes when bidi caret affinity is enabled.

Bug: 894651
Change-Id: I178f1c0b209b907bd6ccad88b3e82d2df60f2907
Reviewed-on: https://chromium-review.googlesource.com/c/1404565
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621990}

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

@@ -239,6 +239,42 @@ NGCaretPosition AdjustCaretPositionForBidiText(
   return BidiAdjustment::AdjustForCaretPositionResolution(caret_position);
 }
 
+bool IsUpstreamAfterLineBreak(const NGCaretPosition& caret_position) {
+  if (caret_position.position_type != NGCaretPositionType::kAtTextOffset)
+    return false;
+
+  DCHECK(caret_position.fragment);
+  DCHECK(caret_position.fragment->PhysicalFragment().IsText());
+  DCHECK(caret_position.text_offset.has_value());
+
+  const NGPhysicalTextFragment& text_fragment =
+      ToNGPhysicalTextFragment(caret_position.fragment->PhysicalFragment());
+  if (!text_fragment.IsLineBreak())
+    return false;
+  return caret_position.text_offset.value() == text_fragment.EndOffset();
+}
+
+NGCaretPosition BetterCandidateBetween(const NGCaretPosition& current,
+                                       const NGCaretPosition& other,
+                                       unsigned offset,
+                                       TextAffinity affinity) {
+  DCHECK(!other.IsNull());
+  if (current.IsNull())
+    return other;
+
+  // There shouldn't be too many cases where we have multiple candidates.
+  // Make sure all of them are captured and handled here.
+
+  // Only known case: either |current| or |other| is upstream after line break.
+  DCHECK_EQ(affinity, TextAffinity::kUpstream);
+  if (IsUpstreamAfterLineBreak(current)) {
+    DCHECK(!IsUpstreamAfterLineBreak(other));
+    return other;
+  }
+  DCHECK(IsUpstreamAfterLineBreak(other));
+  return current;
+}
+
 }  // namespace
 
 // The main function for compute an NGCaretPosition. See the comments at the top
@@ -265,10 +301,8 @@ NGCaretPosition ComputeNGCaretPosition(const LayoutBlockFlow& context,
       return AdjustCaretPositionForBidiText(resolution.caret_position);
 
     DCHECK_EQ(ResolutionType::kFoundCandidate, resolution.type);
-    // TODO(xiaochengh): We are not sure if we can ever find multiple
-    // candidates. Handle it once reached.
-    DCHECK(candidate.IsNull());
-    candidate = resolution.caret_position;
+    candidate = BetterCandidateBetween(candidate, resolution.caret_position,
+                                       offset, affinity);
   }
 
   return AdjustCaretPositionForBidiText(candidate);

third_party/blink/web_tests/FlagExpectations/enable-blink-features=LayoutNG

@@ -288,11 +288,7 @@ crbug.com/591099 tables/mozilla/bugs/bug2973.html [ Pass ]
 crbug.com/591099 virtual/android/rootscroller/set-root-scroller.html [ Pass ]
 crbug.com/591099 virtual/android/rootscroller/set-rootscroller-before-load.html [ Pass ]
 crbug.com/591099 virtual/bidi-caret-affinity/editing/pasteboard/copy-paste-white-space.html [ Pass ]
-crbug.com/591099 virtual/bidi-caret-affinity/editing/selection/click-in-focusable-link-should-not-clear-selection.html [ Crash ]
-crbug.com/591099 virtual/bidi-caret-affinity/editing/selection/doubleclick-whitespace-crash.html [ Crash ]
-crbug.com/591099 virtual/bidi-caret-affinity/editing/selection/legal-positions.html [ Crash ]
 crbug.com/591099 virtual/bidi-caret-affinity/editing/selection/paint-hyphen.html [ Pass ]
-crbug.com/591099 virtual/bidi-caret-affinity/editing/selection/word-granularity.html [ Crash ]
 crbug.com/916511 virtual/composite-after-paint/paint/background/scrolling-background-with-negative-z-child.html [ Failure Crash ]
 crbug.com/591099 virtual/composite-after-paint/paint/invalidation/box/margin.html [ Failure Pass ]
 crbug.com/591099 virtual/display-lock/display-lock/lock-after-append/measure-forced-layout.html [ Failure ]