wm: Fix backdrop showing up when swiping down an ARC app.

I didn't find out the exact cause, but something about the ARC app was
updating the backdrop many times during the course of a swipe, and was
probably creating a new one which did not have an transform or opacity
changes. This happens for regular windows as well, so instead, pause
updates during a home launcher drag.

Test: manual
Bug: 1018125
Change-Id: I1d933921f8c17410fb6a8c81f96c94e93fc59955
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884578Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711035}

ash/wm/workspace/backdrop_controller.cc

@@ -9,6 +9,8 @@
 
 #include "ash/accessibility/accessibility_controller_impl.h"
 #include "ash/accessibility/accessibility_delegate.h"
+#include "ash/home_screen/home_launcher_gesture_handler.h"
+#include "ash/home_screen/home_screen_controller.h"
 #include "ash/public/cpp/app_types.h"
 #include "ash/public/cpp/shell_window_ids.h"
 #include "ash/public/cpp/window_animation_types.h"
@@ -130,10 +132,17 @@ BackdropController::BackdropController(aura::Window* container)
   shell->accessibility_controller()->AddObserver(this);
   shell->wallpaper_controller()->AddObserver(this);
   shell->tablet_mode_controller()->AddObserver(this);
+  shell->home_screen_controller()->home_launcher_gesture_handler()->AddObserver(
+      this);
 }
 
 BackdropController::~BackdropController() {
   auto* shell = Shell::Get();
+  if (shell->home_screen_controller()) {
+    shell->home_screen_controller()
+        ->home_launcher_gesture_handler()
+        ->RemoveObserver(this);
+  }
   // Shell destroys the TabletModeController before destroying all root windows.
   if (shell->tablet_mode_controller())
     shell->tablet_mode_controller()->RemoveObserver(this);
@@ -272,6 +281,18 @@ void BackdropController::OnTabletModeEnded() {
   UpdateBackdrop();
 }
 
+void BackdropController::OnHomeLauncherTargetPositionChanged(
+    bool showing,
+    int64_t display_id) {
+  pause_update_ = true;
+}
+
+void BackdropController::OnHomeLauncherAnimationComplete(bool shown,
+                                                         int64_t display_id) {
+  pause_update_ = false;
+  UpdateBackdrop();
+}
+
 void BackdropController::UpdateBackdropInternal() {
   // Skip the recursive updates.
   if (pause_update_)

ash/wm/workspace/backdrop_controller.h

@@ -9,6 +9,7 @@
 
 #include "ash/accessibility/accessibility_observer.h"
 #include "ash/ash_export.h"
+#include "ash/home_screen/home_launcher_gesture_handler_observer.h"
 #include "ash/public/cpp/tablet_mode_observer.h"
 #include "ash/public/cpp/wallpaper_controller_observer.h"
 #include "ash/public/cpp/window_properties.h"
@@ -41,11 +42,13 @@ namespace ash {
 // 3) In tablet mode:
 //        - Bottom-most snapped window in splitview,
 //        - Top-most activatable window if splitview is inactive.
-class ASH_EXPORT BackdropController : public AccessibilityObserver,
-                                      public OverviewObserver,
-                                      public SplitViewObserver,
-                                      public WallpaperControllerObserver,
-                                      public TabletModeObserver {
+class ASH_EXPORT BackdropController
+    : public AccessibilityObserver,
+      public OverviewObserver,
+      public SplitViewObserver,
+      public WallpaperControllerObserver,
+      public TabletModeObserver,
+      public HomeLauncherGestureHandlerObserver {
  public:
   explicit BackdropController(aura::Window* container);
   ~BackdropController() override;
@@ -90,6 +93,11 @@ class ASH_EXPORT BackdropController : public AccessibilityObserver,
   void OnTabletModeStarted() override;
   void OnTabletModeEnded() override;
 
+  // HomeLauncherGestureHandlerObserver:
+  void OnHomeLauncherTargetPositionChanged(bool showing,
+                                           int64_t display_id) override;
+  void OnHomeLauncherAnimationComplete(bool shown, int64_t display_id) override;
+
  private:
   friend class WorkspaceControllerTestApi;