Fix issue with longpress drag selection motion

If the user has already dragged outside the initial selected word region
by the time the browser gets a selection update, or if the selected word
isn't coincident with the longpress location, the subsequently dragged
selection endpoint can be wrong. For such cases, rather than
using the drag direction for drag anchor picking, just use the distance
between the current touch point and the selection endpoints.

BUG=466749

Review URL: https://codereview.chromium.org/1271823002

Cr-Commit-Position: refs/heads/master@{#341791}
(cherry picked from commit a188bd6)

Review URL: https://codereview.chromium.org/1259593010 .

Cr-Commit-Position: refs/branch-heads/2454@{#228}
Cr-Branched-From: 12bfc33-refs/heads/master@{#338390}

Author: Jared Duke

ui/touch_selection/longpress_drag_selector.cc

@@ -8,6 +8,13 @@
 #include "ui/events/gesture_detection/motion_event.h"
 
 namespace ui {
+namespace {
+
+gfx::Vector2dF SafeNormalize(const gfx::Vector2dF& v) {
+  return v.IsZero() ? v : ScaleVector2d(v, 1.f / v.Length());
+}
+
+}  // namespace
 
 LongPressDragSelector::LongPressDragSelector(
     LongPressDragSelectorClient* client)
@@ -70,21 +77,27 @@ bool LongPressDragSelector::WillHandleTouchEvent(const MotionEvent& event) {
     // If initial motion is up/down, extend the start/end selection bound.
     extend_selection_start = delta.y() < 0;
   } else {
-    // Otherwise extend the selection bound toward which we're moving.
+    // Otherwise extend the selection bound toward which we're moving, or
+    // the closest bound if motion is already away from both bounds.
     // Note that, for mixed RTL text, or for multiline selections triggered
     // by longpress, this may not pick the most suitable drag target
-    gfx::Vector2dF start_delta = selection_start - position;
+    gfx::Vector2dF start_delta = selection_start - longpress_drag_start_anchor_;
+    gfx::Vector2dF end_delta = selection_end - longpress_drag_start_anchor_;
 
     // The vectors must be normalized to make dot product comparison meaningful.
-    if (!start_delta.IsZero())
-      start_delta.Scale(1.f / start_delta.Length());
-    gfx::Vector2dF end_delta = selection_end - position;
-    if (!end_delta.IsZero())
-      end_delta.Scale(1.f / start_delta.Length());
-
-    // The larger the dot product the more similar the direction.
-    extend_selection_start =
-        gfx::DotProduct(start_delta, delta) > gfx::DotProduct(end_delta, delta);
+    gfx::Vector2dF normalized_start_delta = SafeNormalize(start_delta);
+    gfx::Vector2dF normalized_end_delta = SafeNormalize(end_delta);
+    double start_dot_product = gfx::DotProduct(normalized_start_delta, delta);
+    double end_dot_product = gfx::DotProduct(normalized_end_delta, delta);
+
+    if (start_dot_product >= 0 || end_dot_product >= 0) {
+      // The greater the dot product the more similar the direction.
+      extend_selection_start = start_dot_product > end_dot_product;
+    } else {
+      // If we're already moving away from both endpoints, pick the closest.
+      extend_selection_start =
+          start_delta.LengthSquared() < end_delta.LengthSquared();
+    }
   }
 
   gfx::PointF extent = extend_selection_start ? selection_start : selection_end;

ui/touch_selection/longpress_drag_selector_unittest.cc

@@ -139,7 +139,7 @@ TEST_F(LongPressDragSelectorTest, BasicReverseDrag) {
   EXPECT_FALSE(IsDragging());
 
   // Initiate drag motion.
-  EXPECT_TRUE(selector.WillHandleTouchEvent(event.MovePoint(0, 0, 0)));
+  EXPECT_TRUE(selector.WillHandleTouchEvent(event.MovePoint(0, 5, 0)));
   EXPECT_FALSE(IsDragging());
 
   // As the initial motion is leftward, toward the selection start, the
@@ -330,4 +330,38 @@ TEST_F(LongPressDragSelectorTest, SelectionDeactivated) {
   EXPECT_FALSE(selector.WillHandleTouchEvent(event.ReleasePoint()));
 }
 
+TEST_F(LongPressDragSelectorTest, DragFast) {
+  LongPressDragSelector selector(this);
+  MockMotionEvent event;
+
+  // Start a touch sequence.
+  EXPECT_FALSE(selector.WillHandleTouchEvent(event.PressPoint(0, 0)));
+  EXPECT_FALSE(GetAndResetActiveStateChanged());
+
+  // Activate a longpress-triggered selection.
+  gfx::PointF selection_start(0, 10);
+  gfx::PointF selection_end(10, 10);
+  selector.OnLongPressEvent(event.GetEventTime(), gfx::PointF());
+  EXPECT_TRUE(GetAndResetActiveStateChanged());
+  SetSelection(selection_start, selection_end);
+  selector.OnSelectionActivated();
+  EXPECT_FALSE(IsDragging());
+
+  // Initiate drag motion.
+  EXPECT_TRUE(selector.WillHandleTouchEvent(event.MovePoint(0, 15, 5)));
+  EXPECT_FALSE(IsDragging());
+
+  // As the initial motion exceeds both endpoints, the closer bound should
+  // be used for dragging, in this case the selection end.
+  EXPECT_TRUE(selector.WillHandleTouchEvent(
+      event.MovePoint(0, 15.f + kSlop * 2.f, 5.f + kSlop)));
+  EXPECT_TRUE(IsDragging());
+  EXPECT_EQ(selection_end, DragPosition());
+
+  // Release the touch sequence, ending the drag.
+  EXPECT_FALSE(selector.WillHandleTouchEvent(event.ReleasePoint()));
+  EXPECT_FALSE(IsDragging());
+  EXPECT_TRUE(GetAndResetActiveStateChanged());
+}
+
 }  // namespace ui