Reland "ash: Keep active window uncovered on display change"

This is a reland of 92ecc62

Original change's description:
> ash: Keep active window uncovered on display change
> 
> - FocusActivationStore uses ActivateWindow(nullptr) for deactivating
>   to avoid an arbitrary window covering the active window before
>   Restore().
> - RootWindowController puts the moved windows under existing ones
>   on display change.
> 
> Bug: 945754
> Change-Id: I3cacfd3427b8c98c9ce9dd2da05977120c1c1d65
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574824
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#652678}

Bug: 945754
Change-Id: If8b796e8b1736e17e35f34fd2e671689e3a88ed4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577561
Reviewed-by: Mitsuru Oshima (Slow 4/22-26) <oshima@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652931}

ash/display/window_tree_host_manager.cc

@@ -160,12 +160,15 @@ class FocusActivationStore {
     if (active_ && focused_ != active_)
       tracker_.Add(active_);
 
-    // Deactivate the window to close menu / bubble windows.
-    if (clear_focus)
-      activation_client_->DeactivateWindow(active_);
+    // Deactivate the window to close menu / bubble windows. Deactivating by
+    // setting active window to nullptr to avoid side effects of activating an
+    // arbitrary window, such as covering |active_| before Restore().
+    if (clear_focus && active_)
+      activation_client_->ActivateWindow(nullptr);
 
     // Release capture if any.
     capture_client_->SetCapture(nullptr);
+
     // Clear the focused window if any. This is necessary because a
     // window may be deleted when losing focus (fullscreen flash for
     // example).  If the focused window is still alive after move, it'll

ash/root_window_controller.cc

@@ -246,15 +246,19 @@ void ReparentAllWindows(aura::Window* src, aura::Window* dst) {
       // may change as a result of moving other windows.
       const aura::Window::Windows& src_container_children =
           src_container->children();
-      auto iter = src_container_children.begin();
-      while (iter != src_container_children.end() &&
+      auto iter = src_container_children.rbegin();
+      while (iter != src_container_children.rend() &&
              SystemModalContainerLayoutManager::IsModalBackground(*iter)) {
         ++iter;
       }
       // If the entire window list is modal background windows then stop.
-      if (iter == src_container_children.end())
+      if (iter == src_container_children.rend())
         break;
-      ReparentWindow(*iter, dst_container);
+
+      // |iter| is invalidated after ReparentWindow. Cache it to use afterwards.
+      aura::Window* const window = *iter;
+      ReparentWindow(window, dst_container);
+      dst_container->StackChildAtBottom(window);
     }
   }
 }

ash/root_window_controller_unittest.cc

@@ -348,6 +348,50 @@ TEST_F(RootWindowControllerTest, MoveWindows_LockWindowsInUnified) {
   EXPECT_EQ("0,0 600x500", lock_screen->GetNativeWindow()->bounds().ToString());
 }
 
+// Tests that the moved windows (except the active one) are put under existing
+// ones.
+TEST_F(RootWindowControllerTest, MoveWindows_UnderExisting) {
+  UpdateDisplay("600x600,300x300");
+
+  display::Screen* screen = display::Screen::GetScreen();
+  const display::Display primary_display = screen->GetPrimaryDisplay();
+  const display::Display secondary_display = GetSecondaryDisplay();
+
+  views::Widget* existing = CreateTestWidget(gfx::Rect(0, 10, 100, 100));
+  ASSERT_EQ(primary_display.id(),
+            screen->GetDisplayNearestWindow(existing->GetNativeWindow()).id());
+
+  views::Widget* moved = CreateTestWidget(gfx::Rect(650, 10, 100, 100));
+  ASSERT_EQ(secondary_display.id(),
+            screen->GetDisplayNearestWindow(moved->GetNativeWindow()).id());
+
+  views::Widget* active = CreateTestWidget(gfx::Rect(650, 10, 100, 100));
+  ASSERT_TRUE(active->IsActive());
+  ASSERT_EQ(secondary_display.id(),
+            screen->GetDisplayNearestWindow(active->GetNativeWindow()).id());
+
+  // Switch to single display.
+  UpdateDisplay("600x500");
+
+  // |active| is still active.
+  ASSERT_TRUE(active->IsActive());
+
+  // |moved| should be put under |existing|.
+  ASSERT_EQ(moved->GetNativeWindow()->parent(),
+            existing->GetNativeWindow()->parent());
+  bool found_existing = false;
+  bool found_moved = false;
+  for (auto* child : moved->GetNativeWindow()->parent()->children()) {
+    if (child == existing->GetNativeWindow())
+      found_existing = true;
+    if (child == moved->GetNativeWindow()) {
+      found_moved = true;
+      break;
+    }
+  }
+  EXPECT_TRUE(found_moved && !found_existing);
+}
+
 TEST_F(RootWindowControllerTest, ModalContainer) {
   UpdateDisplay("600x600");
   RootWindowController* controller = Shell::GetPrimaryRootWindowController();

ash/wm/overview/overview_session.cc

@@ -650,8 +650,14 @@ void OverviewSession::OnWindowActivating(
     ::wm::ActivationChangeObserver::ActivationReason reason,
     aura::Window* gained_active,
     aura::Window* lost_active) {
-  if (ignore_activations_ || !gained_active ||
-      gained_active == GetOverviewFocusWindow()) {
+  if (ignore_activations_ || gained_active == GetOverviewFocusWindow())
+    return;
+
+  if (!gained_active) {
+    // Cancel overview session and do not restore focus when active window is
+    // set to nullptr. This happens when removing a display.
+    ResetFocusRestoreWindow(false);
+    CancelSelection();
     return;
   }