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

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

Index: chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc
diff --git a/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc b/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc
index 9c9de92b83f593f649bc19a483eaab412db35aef..fe136e25bc9fb47ecac034111fcd4db49161af9d 100644
--- a/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc
+++ b/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) {
+  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->ShelfVisibilityChangesProbablyWithUser(
+             *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;