overview: Show drop target even if split view is disabled

The purpose of the present CL is to add a drop target in a certain
workflow. As of [1], the "drag to snap" mode has become a "normal drag"
mode which is available even when split view mode is not available. For
testing, you can note that in clamshell mode, with the functionality of
[2] disabled (the current default), split view mode is not available,
but you can still do a "normal drag" from overview. There is presently
no drop target in that workflow. There is no drop target depicted in [3]
either, but I am pretty sure a drop target is actually appropriate,
because of [4] (which is assigned to me) if for no other reason.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1612121
[2] https://crbug.com/890029
[3] https://bugs.chromium.org/p/chromium/issues/detail?id=866622#c33
[4] https://crbug.com/961170

Test: manual
Bug: 866622, 961170
Change-Id: I079d01c3e449b10f23a19a9993fbdd44d0c1482e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627562Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663305}

ash/wm/overview/overview_grid.cc

@@ -660,7 +660,6 @@ void OverviewGrid::SetBoundsAndUpdatePositions(
 }
 
 void OverviewGrid::RearrangeDuringDrag(aura::Window* dragged_window,
-                                       const gfx::PointF& location_in_screen,
                                        IndicatorState indicator_state) {
   OverviewItem* drop_target = GetDropTarget();
 
@@ -676,12 +675,11 @@ void OverviewGrid::RearrangeDuringDrag(aura::Window* dragged_window,
     drop_target->SetOpacity(wanted_drop_target_visibility ? 1.f : 0.f);
   }
 
-  OverviewItem* dragged_item = GetOverviewItemContaining(dragged_window);
-
   // Update the grid's bounds.
   const gfx::Rect wanted_grid_bounds =
       GetGridBoundsInScreenDuringDragging(dragged_window, indicator_state);
   if (update_drop_target_visibility || bounds_ != wanted_grid_bounds) {
+    OverviewItem* dragged_item = GetOverviewItemContaining(dragged_window);
     base::flat_set<OverviewItem*> ignored_items;
     if (dragged_item)
       ignored_items.insert(dragged_item);
@@ -689,12 +687,12 @@ void OverviewGrid::RearrangeDuringDrag(aura::Window* dragged_window,
       ignored_items.insert(drop_target);
     SetBoundsAndUpdatePositions(wanted_grid_bounds, ignored_items);
   }
+}
 
-  // If the drop target is on another grid, let that grid handle what follows.
-  if (!drop_target)
-    return;
-
-  // Visually indicate when |dragged_window| is dragged over the drop target.
+void OverviewGrid::UpdateDropTargetBackgroundVisibility(
+    OverviewItem* dragged_item,
+    const gfx::PointF& location_in_screen) {
+  DCHECK(drop_target_widget_);
   aura::Window* target_window =
       GetTargetWindowOnLocation(location_in_screen, dragged_item);
   DropTargetView* drop_target_view =
@@ -749,7 +747,8 @@ void OverviewGrid::OnWindowDragContinued(aura::Window* dragged_window,
                                          IndicatorState indicator_state) {
   DCHECK_EQ(dragged_window->GetRootWindow(), root_window_);
 
-  RearrangeDuringDrag(dragged_window, location_in_screen, indicator_state);
+  RearrangeDuringDrag(dragged_window, indicator_state);
+  UpdateDropTargetBackgroundVisibility(nullptr, location_in_screen);
 
   aura::Window* target_window =
       GetTargetWindowOnLocation(location_in_screen, /*ignored_item=*/nullptr);

ash/wm/overview/overview_grid.h

@@ -135,12 +135,17 @@ class ASH_EXPORT OverviewGrid : public aura::WindowObserver,
       const base::flat_set<OverviewItem*>& ignored_items);
 
   // Updates overview bounds and hides the drop target when a preview area is
-  // shown. Updates the appearance of the drop target to visually indicate when
-  // |dragged_window| is being dragged over it.
+  // shown.
   void RearrangeDuringDrag(aura::Window* dragged_window,
-                           const gfx::PointF& location_in_screen,
                            IndicatorState indicator_state);
 
+  // Updates the appearance of the drop target to visually indicate when the
+  // dragged window is being dragged over it. For dragging from the top, pass
+  // null for |dragged_item|.
+  void UpdateDropTargetBackgroundVisibility(
+      OverviewItem* dragged_item,
+      const gfx::PointF& location_in_screen);
+
   // Shows or hides the selection widget. To be called by an overview item when
   // it is dragged.
   void SetSelectionWidgetVisibility(bool visible);

ash/wm/overview/overview_window_drag_controller.cc

@@ -190,9 +190,9 @@ void OverviewWindowDragController::StartNormalDragMode(
   item_->ScaleUpSelectedItem(
       OVERVIEW_ANIMATION_LAYOUT_OVERVIEW_ITEMS_IN_OVERVIEW);
   original_scaled_size_ = item_->target_bounds().size();
+  item_->overview_grid()->AddDropTargetForDraggingFromOverview(item_);
 
   if (should_allow_split_view_) {
-    item_->overview_grid()->AddDropTargetForDraggingFromOverview(item_);
     overview_session_->SetSplitViewDragIndicatorsIndicatorState(
         CanSnapInSplitview(item_->GetWindow()) ? IndicatorState::kDragArea
                                                : IndicatorState::kCannotSnap,
@@ -252,6 +252,8 @@ void OverviewWindowDragController::ActivateDraggedWindow() {
 
 void OverviewWindowDragController::ResetGesture() {
   if (current_drag_behavior_ == DragBehavior::kNormalDrag) {
+    DCHECK(item_->overview_grid()->drop_target_widget());
+
     Shell::Get()->mouse_cursor_filter()->HideSharedEdgeIndicator();
     item_->DestroyPhantomsForDragging();
     item_->overview_grid()->RemoveDropTarget();
@@ -350,6 +352,8 @@ gfx::RectF OverviewWindowDragController::ContinueNormalDrag(
 
   if (should_allow_split_view_)
     UpdateDragIndicatorsAndOverviewGrid(location_in_screen);
+  item_->overview_grid()->UpdateDropTargetBackgroundVisibility(
+      item_, location_in_screen);
 
   bounds.set_x(centerpoint.x() - bounds.width() / 2.f);
   bounds.set_y(centerpoint.y() - bounds.height() / 2.f);
@@ -360,14 +364,14 @@ OverviewWindowDragController::DragResult
 OverviewWindowDragController::CompleteNormalDrag(
     const gfx::PointF& location_in_screen) {
   DCHECK_EQ(current_drag_behavior_, DragBehavior::kNormalDrag);
+  DCHECK(item_->overview_grid()->drop_target_widget());
   Shell::Get()->mouse_cursor_filter()->HideSharedEdgeIndicator();
   item_->DestroyPhantomsForDragging();
+  item_->overview_grid()->RemoveDropTarget();
 
   const gfx::Point rounded_screen_point =
       gfx::ToRoundedPoint(location_in_screen);
   if (should_allow_split_view_) {
-    DCHECK(item_->overview_grid()->drop_target_widget());
-    item_->overview_grid()->RemoveDropTarget();
     // Update the split view divider bar stuatus if necessary. The divider bar
     // should be placed above the dragged window after drag ends. Note here the
     // passed parameters |snap_position_| and |location_in_screen| won't be used
@@ -441,8 +445,8 @@ void OverviewWindowDragController::UpdateDragIndicatorsAndOverviewGrid(
     snap_position_ = SplitViewController::NONE;
     indicator_state = IndicatorState::kCannotSnap;
   }
-  item_->overview_grid()->RearrangeDuringDrag(
-      item_->GetWindow(), location_in_screen, indicator_state);
+  item_->overview_grid()->RearrangeDuringDrag(item_->GetWindow(),
+                                              indicator_state);
   overview_session_->SetSplitViewDragIndicatorsIndicatorState(indicator_state,
                                                               gfx::Point());
 }