Improve how DocumentMarkerController updates markers in response to text edits

third_party/WebKit/Source/core/editing/markers/DocumentMarker.cpp

Index: third_party/WebKit/Source/core/editing/markers/DocumentMarker.cpp
diff --git a/third_party/WebKit/Source/core/editing/markers/DocumentMarker.cpp b/third_party/WebKit/Source/core/editing/markers/DocumentMarker.cpp
index 05b050da46abe8031ba414e6f9d4e485f18b0d77..a57a48ef25be3e26505d70ea9583cbebabfd343e 100644
--- a/third_party/WebKit/Source/core/editing/markers/DocumentMarker.cpp
+++ b/third_party/WebKit/Source/core/editing/markers/DocumentMarker.cpp
@@ -175,6 +175,52 @@ DocumentMarker::DocumentMarker(const DocumentMarker& marker)
       end_offset_(marker.EndOffset()),
       details_(marker.Details()) {}
 
+Optional<DocumentMarker::MarkerOffsets>
+DocumentMarker::ComputeOffsetsAfterShift(unsigned offset,
+                                         unsigned old_length,
+                                         unsigned new_length) const {
+  MarkerOffsets result;
+  result.start_offset = StartOffset();
+  result.end_offset = EndOffset();
+
+  // algorithm inspired by https://dom.spec.whatwg.org/#concept-cd-replace
+  // but with some changes
+
+  // Deviation from the concept-cd-replace algorithm: second condition in the
+  // next line (don't include text inserted immediately before a marker in the
+  // marked range, but do include the new text if it's replacing text in the
+  // marked range)
+  if (StartOffset() > offset || (StartOffset() == offset && old_length == 0)) {
+    if (StartOffset() <= offset + old_length) {
+      // Marker start was in the replaced text. Move to end of new text
+      // (Deviation from the concept-cd-replace algorithm: that algorithm
+      // would move to the beginning of the new text here)
+      result.start_offset = offset + new_length;
+    } else {
+      // Marker start was after the replaced text. Shift by length
+      // difference
+      result.start_offset = StartOffset() + new_length - old_length;
+    }
+  }
+
+  if (EndOffset() > offset) {
+    // Deviation from the concept-cd-replace algorithm: < instead of <= in
+    // the next line
+    if (EndOffset() < offset + old_length) {
+      // Marker end was in the replaced text. Move to beginning of new text
+      result.end_offset = offset;
+    } else {
+      // Marker end was after the replaced text. Shift by length difference
+      result.end_offset = EndOffset() + new_length - old_length;
+    }
+  }
+
+  if (result.start_offset >= result.end_offset)
+    return WTF::kNullopt;
+
+  return result;
+}
+
 void DocumentMarker::ShiftOffsets(int delta) {
   start_offset_ += delta;
   end_offset_ += delta;