Ease selection dragging between large/small text lines

ui/touch_selection/touch_selection_controller.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/touch_selection/touch_selection_controller.h"
 
 #include "base/auto_reset.h"
 #include "base/logging.h"
 
 namespace ui {
 namespace {
 
+gfx::Vector2dF ComputeLineOffsetFromBottom(const SelectionBound& bound) {
+  gfx::Vector2dF line_offset =
+      gfx::ScaleVector2d(bound.edge_top() - bound.edge_bottom(), 0.5f);
+  // An offset of 5 DIPs is sufficient for most line sizes. For small lines,
+  // using half the line height avoids synthesizing a point on a line above
+  // (or below) the intended line.
+  const gfx::Vector2dF kMaxLineOffset(5.f, 5.f);
+  line_offset.SetToMin(kMaxLineOffset);
+  line_offset.SetToMax(-kMaxLineOffset);
+  return line_offset;
+}
+
 TouchHandleOrientation ToTouchHandleOrientation(SelectionBound::Type type) {
   switch (type) {
     case SelectionBound::LEFT:
       return TOUCH_HANDLE_LEFT;
     case SelectionBound::RIGHT:
       return TOUCH_HANDLE_RIGHT;
     case SelectionBound::CENTER:
       return TOUCH_HANDLE_CENTER;
     case SelectionBound::EMPTY:
       return TOUCH_HANDLE_ORIENTATION_UNDEFINED;
@@ skipping 209 matching lines @@
   // so we must make sure that the base point is set correctly.
   client_->SelectBetweenCoordinates(base, extent);
 
   client_->OnSelectionEvent(SELECTION_DRAG_STARTED, handle.position());
 }
 
 void TouchSelectionController::OnHandleDragUpdate(const TouchHandle& handle,
                                                   const gfx::PointF& position) {
   // As the position corresponds to the bottom left point of the selection
   // bound, offset it by half the corresponding line height.
-  gfx::Vector2dF line_offset = &handle == end_selection_handle_.get()
+  gfx::Vector2dF line_offset = &handle == start_selection_handle_.get()
                                    ? GetStartLineOffset()
                                    : GetEndLineOffset();
   gfx::PointF line_position = position + line_offset;
   if (&handle == insertion_handle_.get()) {
     client_->MoveCaret(line_position);
   } else {
     client_->MoveRangeSelectionExtent(line_position);
   }
 }
 
@@ skipping 148 matching lines @@
 
 const gfx::PointF& TouchSelectionController::GetStartPosition() const {
   return start_.edge_bottom();
 }
 
 const gfx::PointF& TouchSelectionController::GetEndPosition() const {
   return end_.edge_bottom();
 }
 
 gfx::Vector2dF TouchSelectionController::GetStartLineOffset() const {
-  return gfx::ScaleVector2d(start_.edge_top() - start_.edge_bottom(), 0.5f);
+  return ComputeLineOffsetFromBottom(start_);
 }
 
 gfx::Vector2dF TouchSelectionController::GetEndLineOffset() const {
-  return gfx::ScaleVector2d(end_.edge_top() - end_.edge_bottom(), 0.5f);
+  return ComputeLineOffsetFromBottom(end_);
 }
 
 bool TouchSelectionController::GetStartVisible() const {
   return start_.visible() && !temporarily_hidden_;
 }
 
 bool TouchSelectionController::GetEndVisible() const {
   return end_.visible() && !temporarily_hidden_;
 }
 
 TouchHandle::AnimationStyle TouchSelectionController::GetAnimationStyle(
     bool was_active) const {
   return was_active && client_->SupportsAnimation()
              ? TouchHandle::ANIMATION_SMOOTH
              : TouchHandle::ANIMATION_NONE;
 }
 
 }  // namespace ui