Avoiding double resize of maximized windows upon user switch due to showing and hiding of the shelf

BUG=385992
TEST=MultiUserWindowManagerChromeOSTest.TestBlackBarCover & visual inspection

Review URL: https://codereview.chromium.org/448493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287880 0039d316-1c4b-4281-b951-d872f2087c98

ash/shelf/shelf_widget.cc

@@ -325,6 +325,7 @@ class ShelfWidget::DelegateView : public views::WidgetDelegate,
   FocusCycler* focus_cycler() { return focus_cycler_; }
 
   ui::Layer* opaque_background() { return &opaque_background_; }
+  ui::Layer* opaque_foreground() { return &opaque_foreground_; }
 
   // Set if the shelf area is dimmed (eg when a window is maximized).
   void SetDimmed(bool dimmed);
@@ -383,7 +384,12 @@ class ShelfWidget::DelegateView : public views::WidgetDelegate,
   scoped_ptr<views::Widget> dimmer_;
   FocusCycler* focus_cycler_;
   int alpha_;
+  // A black background layer which is shown when a maximized window is visible.
   ui::Layer opaque_background_;
+  // A black foreground layer which is shown while transitioning between users.
+  // Note: Since the back- and foreground layers have different functions they
+  // can be used simultaneously - so no repurposing possible.
+  ui::Layer opaque_foreground_;
 
   // The view which does the dimming.
   DimmerView* dimmer_view_;
@@ -399,12 +405,16 @@ ShelfWidget::DelegateView::DelegateView(ShelfWidget* shelf)
       focus_cycler_(NULL),
       alpha_(0),
       opaque_background_(ui::LAYER_SOLID_COLOR),
+      opaque_foreground_(ui::LAYER_SOLID_COLOR),
       dimmer_view_(NULL),
       disable_dimming_animations_for_test_(false) {
   set_allow_deactivate_on_esc(true);
   opaque_background_.SetColor(SK_ColorBLACK);
   opaque_background_.SetBounds(GetLocalBounds());
   opaque_background_.SetOpacity(0.0f);
+  opaque_foreground_.SetColor(SK_ColorBLACK);
+  opaque_foreground_.SetBounds(GetLocalBounds());
+  opaque_foreground_.SetOpacity(0.0f);
 }
 
 ShelfWidget::DelegateView::~DelegateView() {
@@ -450,6 +460,7 @@ bool ShelfWidget::DelegateView::GetDimmed() const {
 
 void ShelfWidget::DelegateView::SetParentLayer(ui::Layer* layer) {
   layer->Add(&opaque_background_);
+  layer->Add(&opaque_foreground_);
   ReorderLayers();
 }
 
@@ -556,10 +567,12 @@ void ShelfWidget::DelegateView::Layout() {
 void ShelfWidget::DelegateView::ReorderChildLayers(ui::Layer* parent_layer) {
   views::View::ReorderChildLayers(parent_layer);
   parent_layer->StackAtBottom(&opaque_background_);
+  parent_layer->StackAtTop(&opaque_foreground_);
 }
 
 void ShelfWidget::DelegateView::OnBoundsChanged(const gfx::Rect& old_bounds) {
   opaque_background_.SetBounds(GetLocalBounds());
+  opaque_foreground_.SetBounds(GetLocalBounds());
   if (dimmer_)
     dimmer_->SetBounds(GetBoundsInScreen());
 }
@@ -678,6 +691,27 @@ ShelfBackgroundType ShelfWidget::GetBackgroundType() const {
   return SHELF_BACKGROUND_DEFAULT;
 }
 
+void ShelfWidget::HideShelfBehindBlackBar(bool hide, int animation_time_ms) {
+  if (IsShelfHiddenBehindBlackBar() == hide)
+    return;
+
+  ui::Layer* opaque_foreground = delegate_view_->opaque_foreground();
+  float target_opacity = hide ? 1.0f : 0.0f;
+  scoped_ptr<ui::ScopedLayerAnimationSettings> opaque_foreground_animation;
+  opaque_foreground_animation.reset(new ui::ScopedLayerAnimationSettings(
+      opaque_foreground->GetAnimator()));
+  opaque_foreground_animation->SetTransitionDuration(
+      base::TimeDelta::FromMilliseconds(animation_time_ms));
+  opaque_foreground_animation->SetPreemptionStrategy(
+      ui::LayerAnimator::REPLACE_QUEUED_ANIMATIONS);
+
+  opaque_foreground->SetOpacity(target_opacity);
+}
+
+bool ShelfWidget::IsShelfHiddenBehindBlackBar() const {
+  return delegate_view_->opaque_foreground()->GetTargetOpacity() != 0.0f;
+}
+
 // static
 bool ShelfWidget::ShelfAlignmentAllowed() {
   user::LoginStatus login_status =

ash/shelf/shelf_widget.h

@@ -45,6 +45,11 @@ class ASH_EXPORT ShelfWidget : public views::Widget,
                            BackgroundAnimatorChangeType change_type);
   ShelfBackgroundType GetBackgroundType() const;
 
+  // Hide the shelf behind a black bar during e.g. a user transition when |hide|
+  // is true. The |animation_time_ms| will be used as animation duration.
+  void HideShelfBehindBlackBar(bool hide, int animation_time_ms);
+  bool IsShelfHiddenBehindBlackBar() const;
+
   // Causes shelf items to be slightly dimmed (e.g. when a window is maximized).
   void SetDimsShelf(bool dimming);
   bool GetDimsShelf() const;

chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc

@@ -197,6 +197,48 @@ std::string GetPrefForRootWindow(PrefService* pref_service,
   return default_string;
 }
 
+// Gets the shelf auto hide behavior from prefs for a root window.
+ash::ShelfAutoHideBehavior GetShelfAutoHideBehaviorFromPrefs(
+    Profile* profile,
+    aura::Window* root_window) {
+  // Don't show the shelf in app mode.
+  if (chrome::IsRunningInAppMode())
+    return ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN;
+
+  // See comment in |kShelfAlignment| as to why we consider two prefs.
+  const std::string behavior_value(
+      GetPrefForRootWindow(profile->GetPrefs(),
+                           root_window,
+                           prefs::kShelfAutoHideBehaviorLocal,
+                           prefs::kShelfAutoHideBehavior));
+
+  // Note: To maintain sync compatibility with old images of chrome/chromeos
+  // the set of values that may be encountered includes the now-extinct
+  // "Default" as well as "Never" and "Always", "Default" should now
+  // be treated as "Never" (http://crbug.com/146773).
+  if (behavior_value == ash::kShelfAutoHideBehaviorAlways)
+    return ash::SHELF_AUTO_HIDE_BEHAVIOR_ALWAYS;
+  return ash::SHELF_AUTO_HIDE_BEHAVIOR_NEVER;
+}
+
+// Gets the shelf alignment from prefs for a root window.
+ash::ShelfAlignment GetShelfAlignmentFromPrefs(Profile* profile,
+                                               aura::Window* root_window) {
+  // See comment in |kShelfAlignment| as to why we consider two prefs.
+  const std::string alignment_value(
+      GetPrefForRootWindow(profile->GetPrefs(),
+                           root_window,
+                           prefs::kShelfAlignmentLocal,
+                           prefs::kShelfAlignment));
+  if (alignment_value == ash::kShelfAlignmentLeft)
+    return ash::SHELF_ALIGNMENT_LEFT;
+  else if (alignment_value == ash::kShelfAlignmentRight)
+    return ash::SHELF_ALIGNMENT_RIGHT;
+  else if (alignment_value == ash::kShelfAlignmentTop)
+    return ash::SHELF_ALIGNMENT_TOP;
+  return ash::SHELF_ALIGNMENT_BOTTOM;
+}
+
 // If prefs have synced and no user-set value exists at |local_path|, the value
 // from |synced_path| is copied to |local_path|.
 void MaybePropagatePrefToLocal(PrefServiceSyncable* pref_service,
@@ -920,24 +962,7 @@ Profile* ChromeLauncherController::profile() {
 
 ash::ShelfAutoHideBehavior ChromeLauncherController::GetShelfAutoHideBehavior(
     aura::Window* root_window) const {
-  // Don't show the shelf in app mode.
-  if (chrome::IsRunningInAppMode())
-    return ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN;
-
-  // See comment in |kShelfAlignment| as to why we consider two prefs.
-  const std::string behavior_value(
-      GetPrefForRootWindow(profile_->GetPrefs(),
-                           root_window,
-                           prefs::kShelfAutoHideBehaviorLocal,
-                           prefs::kShelfAutoHideBehavior));
-
-  // Note: To maintain sync compatibility with old images of chrome/chromeos
-  // the set of values that may be encountered includes the now-extinct
-  // "Default" as well as "Never" and "Always", "Default" should now
-  // be treated as "Never" (http://crbug.com/146773).
-  if (behavior_value == ash::kShelfAutoHideBehaviorAlways)
-    return ash::SHELF_AUTO_HIDE_BEHAVIOR_ALWAYS;
-  return ash::SHELF_AUTO_HIDE_BEHAVIOR_NEVER;
+  return GetShelfAutoHideBehaviorFromPrefs(profile_, root_window);
 }
 
 bool ChromeLauncherController::CanUserModifyShelfAutoHideBehavior(
@@ -1438,6 +1463,25 @@ bool ChromeLauncherController::IsBrowserFromActiveUser(Browser* browser) {
   return multi_user_util::IsProfileFromActiveUser(browser->profile());
 }
 
+bool ChromeLauncherController::ShelfBoundsChangesProbablyWithUser(
+    aura::Window* root_window,
+    const std::string& user_id) const {
+  Profile* other_profile = multi_user_util::GetProfileFromUserID(user_id);
+  DCHECK_NE(other_profile, profile_);
+
+  // Note: The Auto hide state from preferences is not the same as the actual
+  // visibility of the shelf. Depending on all the various states (full screen,
+  // no window on desktop, multi user, ..) the shelf could be shown - or not.
+  bool currently_shown = ash::SHELF_AUTO_HIDE_BEHAVIOR_NEVER ==
+      GetShelfAutoHideBehaviorFromPrefs(profile_, root_window);
+  bool other_shown = ash::SHELF_AUTO_HIDE_BEHAVIOR_NEVER ==
+      GetShelfAutoHideBehaviorFromPrefs(other_profile, root_window);
+
+  return currently_shown != other_shown ||
+         GetShelfAlignmentFromPrefs(profile_, root_window) !=
+             GetShelfAlignmentFromPrefs(other_profile, root_window);
+}
+
 void ChromeLauncherController::LauncherItemClosed(ash::ShelfID id) {
   IDToItemControllerMap::iterator iter = id_to_item_controller_map_.find(id);
   CHECK(iter != id_to_item_controller_map_.end());
@@ -1696,20 +1740,8 @@ void ChromeLauncherController::SetShelfAlignmentFromPrefs() {
 
   for (aura::Window::Windows::const_iterator iter = root_windows.begin();
        iter != root_windows.end(); ++iter) {
-    // See comment in |kShelfAlignment| as to why we consider two prefs.
-    const std::string alignment_value(
-        GetPrefForRootWindow(profile_->GetPrefs(),
-                             *iter,
-                             prefs::kShelfAlignmentLocal,
-                             prefs::kShelfAlignment));
-    ash::ShelfAlignment alignment = ash::SHELF_ALIGNMENT_BOTTOM;
-    if (alignment_value == ash::kShelfAlignmentLeft)
-      alignment = ash::SHELF_ALIGNMENT_LEFT;
-    else if (alignment_value == ash::kShelfAlignmentRight)
-      alignment = ash::SHELF_ALIGNMENT_RIGHT;
-    else if (alignment_value == ash::kShelfAlignmentTop)
-      alignment = ash::SHELF_ALIGNMENT_TOP;
-    ash::Shell::GetInstance()->SetShelfAlignment(alignment, *iter);
+    ash::Shell::GetInstance()->SetShelfAlignment(
+        GetShelfAlignmentFromPrefs(profile_, *iter), *iter);
   }
 }
 

chrome/browser/ui/ash/launcher/chrome_launcher_controller.h

@@ -384,6 +384,13 @@ class ChromeLauncherController : public ash::ShelfDelegate,
   // Returns true if |browser| is owned by the active user.
   bool IsBrowserFromActiveUser(Browser* browser);
 
+  // Check if the shelf visibility (location, visibility) will change with a new
+  // user profile or not. However, since the full visibility calculation of the
+  // shelf cannot be performed here, this is only a probability used for
+  // animation predictions.
+  bool ShelfBoundsChangesProbablyWithUser(aura::Window* root_window,
+                                          const std::string& user_id) const;
+
   // Access to the BrowserStatusMonitor for tests.
   BrowserStatusMonitor* browser_status_monitor_for_test() {
     return browser_status_monitor_.get();

chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos_unittest.cc

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "ash/root_window_controller.h"
+#include "ash/shelf/shelf_widget.h"
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/test/ash_test_base.h"
@@ -768,6 +770,9 @@ TEST_F(MultiUserWindowManagerChromeOSTest, AnimationSteps) {
             ash::Shell::GetInstance()->GetShelfAutoHideBehavior(
                 window(0)->GetRootWindow()));
   EXPECT_EQ(1.0f, window(0)->layer()->GetTargetOpacity());
+  ash::ShelfWidget* shelf = ash::RootWindowController::ForWindow(
+      window(0))->shelf();
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
 
   // Start the animation and see that the old window is becoming invisible, the
   // new one visible, the background starts transitionining and the shelf hides.
@@ -779,6 +784,7 @@ TEST_F(MultiUserWindowManagerChromeOSTest, AnimationSteps) {
   EXPECT_EQ(ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN,
             ash::Shell::GetInstance()->GetShelfAutoHideBehavior(
                 window(0)->GetRootWindow()));
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
 
   // Staring the next step should show the shelf again, but there are many
   // subsystems missing (preferences system, ChromeLauncherController, ...)
@@ -790,9 +796,11 @@ TEST_F(MultiUserWindowManagerChromeOSTest, AnimationSteps) {
   EXPECT_EQ(ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN,
             ash::Shell::GetInstance()->GetShelfAutoHideBehavior(
                 window(0)->GetRootWindow()));
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
 
   // After the finalize the animation of the wallpaper should be finished.
   AdvanceUserTransitionAnimation();
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
   EXPECT_EQ("B", GetWallaperUserIdForTest());
 }
 
@@ -990,5 +998,86 @@ TEST_F(MultiUserWindowManagerChromeOSTest, ShowForUserSwitchesDesktop) {
   EXPECT_EQ("H[a,b], H[b], S[c]", GetStatus());
 }
 
+class TestWindowObserver : public aura::WindowObserver {
+ public:
+  TestWindowObserver(): resize_calls_(0) {}
+  virtual ~TestWindowObserver() {};
+
+  virtual void OnWindowBoundsChanged(aura::Window* window,
+                                     const gfx::Rect& old_bounds,
+                                     const gfx::Rect& new_bounds) OVERRIDE {
+    resize_calls_++;
+  }
+
+  int resize_calls() { return resize_calls_; }
+
+ private:
+  int resize_calls_;
+
+  DISALLOW_COPY_AND_ASSIGN(TestWindowObserver);
+};
+
+// Test that switching between users with the shelf in the same place, the shelf
+// will get covered with a black bar instead being hidden and re-shown.
+TEST_F(MultiUserWindowManagerChromeOSTest, TestBlackBarCover) {
+  SetUpForThisManyWindows(2);
+  multi_user_window_manager()->SetWindowOwner(window(0), "A");
+  multi_user_window_manager()->SetWindowOwner(window(1), "B");
+  aura::Window* root_window = ash::Shell::GetInstance()->GetPrimaryRootWindow();
+
+  // Turn the use of delays and animation on.
+  multi_user_window_manager()->SetAnimationSpeedForTest(
+      chrome::MultiUserWindowManagerChromeOS::ANIMATION_SPEED_FAST);
+  EXPECT_NE(ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN,
+            ash::Shell::GetInstance()->GetShelfAutoHideBehavior(root_window));
+  ash::ShelfWidget* shelf = ash::RootWindowController::ForWindow(
+      root_window)->shelf();
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
+
+  // First test that with no maximized window we show/hide the shelf.
+  StartUserTransitionAnimation("B");
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
+  EXPECT_EQ(ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN,
+            ash::Shell::GetInstance()->GetShelfAutoHideBehavior(root_window));
+
+  // Staring the next step should show the shelf again.
+  AdvanceUserTransitionAnimation();
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
+
+  AdvanceUserTransitionAnimation();
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
+  ash::Shell::GetInstance()->SetShelfAutoHideBehavior(
+      ash::SHELF_AUTO_HIDE_BEHAVIOR_NEVER, root_window);
+
+  // Now we maximize the windows which will cause the black overlay to show up.
+  wm::GetWindowState(window(0))->Maximize();
+  wm::GetWindowState(window(1))->Maximize();
+  // We set a window observer on both windows to see that no resize is performed
+  // on our test windows.
+  TestWindowObserver window_observer;
+  window(0)->AddObserver(&window_observer);
+  window(1)->AddObserver(&window_observer);
+
+  // Start the animation and see that the shelf gets hidden by the black bar,
+  // and the AutoHide behavior remains as it was.
+  StartUserTransitionAnimation("A");
+  EXPECT_TRUE(shelf->IsShelfHiddenBehindBlackBar());
+  EXPECT_NE(ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN,
+            ash::Shell::GetInstance()->GetShelfAutoHideBehavior(root_window));
+
+  // Staring the next step should show the shelf again.
+  AdvanceUserTransitionAnimation();
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
+  EXPECT_NE(ash::SHELF_AUTO_HIDE_ALWAYS_HIDDEN,
+            ash::Shell::GetInstance()->GetShelfAutoHideBehavior(root_window));
+
+  AdvanceUserTransitionAnimation();
+  EXPECT_FALSE(shelf->IsShelfHiddenBehindBlackBar());
+  window(0)->RemoveObserver(&window_observer);
+  window(1)->RemoveObserver(&window_observer);
+  // No resize should have been done to the window.
+  EXPECT_EQ(0, window_observer.resize_calls());
+}
+
 }  // namespace test
 }  // namespace ash

chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc

@@ -7,6 +7,7 @@
 #include "ash/desktop_background/user_wallpaper_delegate.h"
 #include "ash/root_window_controller.h"
 #include "ash/shelf/shelf_layout_manager.h"
+#include "ash/shelf/shelf_widget.h"
 #include "ash/shell.h"
 #include "ash/wm/mru_window_tracker.h"
 #include "ash/wm/window_positioner.h"
@@ -53,7 +54,7 @@ UserSwichAnimatorChromeOS::UserSwichAnimatorChromeOS(
       new_user_id_(new_user_id),
       animation_speed_ms_(animation_speed_ms),
       animation_step_(ANIMATION_STEP_HIDE_OLD_USER),
-      screen_cover_(GetScreenCover()) {
+      screen_cover_(GetScreenCover(NULL)) {
   AdvanceUserTransitionAnimation();
 
   if (!animation_speed_ms_) {
@@ -156,21 +157,33 @@ void UserSwichAnimatorChromeOS::TransitionWallpaper(
 
 void UserSwichAnimatorChromeOS::TransitionUserShelf(
     AnimationStep animation_step) {
+  ChromeLauncherController* chrome_launcher_controller =
+      ChromeLauncherController::instance();
   // The shelf animation duration override.
   int duration_override = animation_speed_ms_;
   // Handle the shelf order of items. This is done once the old user is hidden.
   if (animation_step == ANIMATION_STEP_SHOW_NEW_USER) {
     // Some unit tests have no ChromeLauncherController.
-    if (ChromeLauncherController::instance())
-      ChromeLauncherController::instance()->ActiveUserChanged(new_user_id_);
-    // We kicked off the shelf animation in the command above. As such we can
-    // disable the override now again.
+    if (chrome_launcher_controller)
+      chrome_launcher_controller->ActiveUserChanged(new_user_id_);
+    // Hide the black rectangle on top of each shelf again.
+    aura::Window::Windows root_windows = ash::Shell::GetAllRootWindows();
+    for (aura::Window::Windows::const_iterator iter = root_windows.begin();
+         iter != root_windows.end(); ++iter) {
+      ash::ShelfWidget* shelf =
+          ash::RootWindowController::ForWindow(*iter)->shelf();
+      shelf->HideShelfBehindBlackBar(false, duration_override);
+    }
+    // We kicked off the shelf animation above and the override can be
+    // removed.
     duration_override = 0;
   }
 
   if (!animation_speed_ms_ || animation_step == ANIMATION_STEP_FINALIZE)
     return;
 
+  // Note: The animation duration override will be set before the old user gets
+  // hidden and reset after the animations for the new user got kicked off.
   ash::Shell::RootWindowControllerList controller =
       ash::Shell::GetInstance()->GetAllRootWindowControllers();
   for (ash::Shell::RootWindowControllerList::iterator iter = controller.begin();
@@ -179,11 +192,28 @@ void UserSwichAnimatorChromeOS::TransitionUserShelf(
         duration_override);
   }
 
+  if (animation_step != ANIMATION_STEP_HIDE_OLD_USER)
+    return;
+
   // For each root window hide the shelf.
-  if (animation_step == ANIMATION_STEP_HIDE_OLD_USER) {
   aura::Window::Windows root_windows = ash::Shell::GetAllRootWindows();
+
   for (aura::Window::Windows::const_iterator iter = root_windows.begin();
        iter != root_windows.end(); ++iter) {
+    // Hiding the shelf will cause a resize on a maximized window.
+    // If the shelf is then shown for the following user in the same location,
+    // the window gets resized again. Since each resize can cause a considerable
+    // CPU usage and therefore effect jank, we should avoid hiding the shelf if
+    // the start and end location are the same and cover the shelf instead with
+    // a black rectangle on top.
+    if (GetScreenCover(*iter) != NO_USER_COVERS_SCREEN &&
+        (!chrome_launcher_controller ||
+         !chrome_launcher_controller->ShelfBoundsChangesProbablyWithUser(
+             *iter, new_user_id_))) {
+      ash::ShelfWidget* shelf =
+          ash::RootWindowController::ForWindow(*iter)->shelf();
+      shelf->HideShelfBehindBlackBar(true, duration_override);
+    } else {
       // This shelf change is only part of the animation and will be updated by
       // ChromeLauncherController::ActiveUserChanged() to the new users value.
       // Note that the user preference will not be changed.
@@ -300,13 +330,15 @@ void UserSwichAnimatorChromeOS::TransitionWindows(
 }
 
 UserSwichAnimatorChromeOS::TransitioningScreenCover
-UserSwichAnimatorChromeOS::GetScreenCover() {
+UserSwichAnimatorChromeOS::GetScreenCover(aura::Window* root_window) {
   TransitioningScreenCover cover = NO_USER_COVERS_SCREEN;
   for (MultiUserWindowManagerChromeOS::WindowToEntryMap::const_iterator it_map =
            owner_->window_to_entry().begin();
        it_map != owner_->window_to_entry().end();
        ++it_map) {
     aura::Window* window = it_map->first;
+    if (root_window && window->GetRootWindow() != root_window)
+      continue;
     if (window->IsVisible() && CoversScreen(window)) {
       if (cover == NEW_USER_COVERS_SCREEN)
         return BOTH_USERS_COVER_SCREEN;

chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h

@@ -84,8 +84,9 @@ class UserSwichAnimatorChromeOS {
   void TransitionWindows(AnimationStep animation_step);
 
   // Check if a window is maximized / fullscreen / covering the entire screen.
-  // TODO(skuhne): We might want to do this on a per screen basis.
-  TransitioningScreenCover GetScreenCover();
+  // If a |root_window| is given, the screen coverage of that root_window is
+  // tested, otherwise all screens.
+  TransitioningScreenCover GetScreenCover(aura::Window* root_window);
 
   // The owning window manager.
   MultiUserWindowManagerChromeOS* owner_;