Tune tap disambiguation triggering threshold.

third_party/WebKit/Source/core/page/TouchDisambiguation.cpp

 /*
  * Copyright (C) 2012 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 134 matching lines @@
             touch_targets.insert(&node, TouchTargetData()).stored_value->value;
         target_data.window_bounding_box = BoundingBoxForEventNodes(&node);
         target_data.score = ScoreTouchTarget(touch_point, touch_point_padding,
                                              target_data.window_bounding_box);
         best_score = std::max(best_score, target_data.score);
         break;
       }
     }
   }
 
+  // The scoring function uses the overlap area with the fat point as the score.
+  // We ignore the candidates that have less than this (empirically tuned)
+  // fraction of overlap than the best candidate to avoid excessive popups.
+  //
+  // If this value were 1, then the disambiguation feature would only be seen
+  // when two nodes have precisely the same overlap with the touch radius.  If
+  // it were 0, then any miniscule overlap with the edge of another node would
+  // trigger it.
+  const float kRelativeAmbiguityThreshold = 0.75f;
+
   for (const auto& touch_target : touch_targets) {
-    // Currently the scoring function uses the overlap area with the fat point
-    // as the score.  We ignore the candidates that has less than 1/2 overlap
-    // (we consider not really ambiguous enough) than the best candidate to
-    // avoid excessive popups.
-    if (touch_target.value.score < best_score * 0.5)
+    if (touch_target.value.score < best_score * kRelativeAmbiguityThreshold)
       continue;
     good_targets.push_back(touch_target.value.window_bounding_box);
     highlight_nodes.push_back(touch_target.key);
   }
 }
 
 }  // namespace blink