Revert "Clamshell <-> tablet mode transition"

This reverts commit 87d6424d.

Reason for revert: suspect causing ash_unittests failure on Linux Chromium OS ASan LSan Tests (1).
https://ci.chromium.org/p/chromium/builders/ci/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%281%29/34147
deterministic failures:
TabletModeWindowManagerWithClamshellSplitViewTest.ClamshellTabletTransitionTest

==23210==ERROR: AddressSanitizer: container-overflow on address 0x6020000e12b8 at pc 0x55b059128cb9 bp 0x7ffdb6f1b8d0 sp 0x7ffdb6f1b8c8
READ of size 8 at 0x6020000e12b8 thread T0
    #0 0x55b059128cb8 in ash::SplitViewDivider::~SplitViewDivider() ./../../ash/wm/splitview/split_view_divider.cc:253:19
    #1 0x55b059128f4d in ash::SplitViewDivider::~SplitViewDivider() ./../../ash/wm/splitview/split_view_divider.cc:249:39
    #2 0x55b0591499fc in ash::TabletModeController::SetTabletModeEnabledInternal(bool) ./../../ash/wm/tablet_mode/tablet_mode_controller.cc:622:16
    #3 0x55b052f19119 in ash::TabletModeWindowManagerTest::DestroyTabletModeWindowManager() ./../../ash/wm/tablet_mode/tablet_mode_window_manager_unittest.cc:138:45
    #4 0x55b052f8060b in ash::TabletModeWindowManagerWithClamshellSplitViewTest_ClamshellTabletTransitionTest_Test::TestBody() ./../../ash/wm/tablet_mode/tablet_mode_window_manager_unittest.cc:1974:3

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8909703871929620480/+/steps/ash_unittests/0/logs/Deterministic_failure:_TabletModeWindowManagerWithClamshellSplitViewTest.ClamshellTabletTransitionTest__status_CRASH_/0

Original change's description:
> Clamshell <-> tablet mode transition
> 
> There are 3 cases in clamshell to tablet mode transition:
> 1) overview is active but splitview is not: keep overview active during
>    transition.
> 2) overview and splitview are both active: keep them both active during
>    transition.
> 3) overview and splitview are both inactive: keep the current behavior.
>    a) if the top window is a snapped window, put it in splitview
>    b) if the second top window is also a snapped window that snaps to the
>       opposite side of the screen, put it in splitview as well
>    c) if the top window is not a snapped window, maximize or fullscreen
>       all windows when entering tablet mode.
> 
> There are 4 cases in tablet to clamshell mode transition:
> 1) overview is active but splitview is not: keep overview active during
>    transition.
> 2) overview and splitview are both active: keep them both active during
>    transition.
> 3) overview is inactive but splitview is active (i.e., two snapped windows),
>    splitview will be no longer active after transition. But the two snapped
>    window will still keep snapped in clamshell mode.
> 4) overview and splitview are both inactive: keep the current behavior,
>    i.e., restore all windows to its old window state before entering tablet
>    mode.
> 
> 
> Bug: 890029
> Change-Id: I7769021f74c3e9d591b19258be2821992d780451
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1658955
> Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#670678}

TBR=oshima@chromium.org,xdai@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 890029
Change-Id: I518170a62a0cf82aff14748679e3a5f12f054f72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675924Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#671997}

ash/wm/splitview/split_view_controller.cc

@@ -768,10 +768,10 @@ void SplitViewController::OnWindowBoundsChanged(
     const gfx::Rect& new_bounds,
     ui::PropertyChangeReason reason) {
   if (split_view_type_ != SplitViewType::kClamshellType ||
-      reason == ui::PropertyChangeReason::FROM_ANIMATION ||
-      !InSplitViewMode()) {
+      reason == ui::PropertyChangeReason::FROM_ANIMATION) {
     return;
   }
+  DCHECK(InSplitViewMode());
 
   wm::WindowState* window_state = wm::GetWindowState(window);
   const bool is_window_moved = window_state->is_dragged() &&
@@ -1109,37 +1109,14 @@ void SplitViewController::OnDisplayMetricsChanged(
   UpdateSnappedWindowsAndDividerBounds();
 }
 
-void SplitViewController::OnTabletModeStarting() {
+void SplitViewController::OnTabletModeStarted() {
   split_view_type_ = SplitViewType::kTabletType;
-
-  // If splitview is active when tablet mode is starting, do the clamshell mode
-  // splitview to tablet mode splitview transition by adding the split view
-  // divider bar.
-  if (InSplitViewMode()) {
-    divider_position_ = GetClosestFixedDividerPosition();
-    split_view_divider_ = std::make_unique<SplitViewDivider>(
-        this, GetDefaultSnappedWindow()->GetRootWindow());
-    UpdateSnappedWindowsAndDividerBounds();
-    NotifyDividerPositionChanged();
-  }
 }
 
 void SplitViewController::OnTabletModeEnding() {
-  if (IsClamshellSplitViewModeEnabled()) {
+  if (IsClamshellSplitViewModeEnabled())
     split_view_type_ = SplitViewType::kClamshellType;
-
-    // If splitview is active when tablet mode is ending, simply destroy the
-    // split view divider bar as we don't have the bar in clamshell split view
-    // mode.
-    if (InSplitViewMode())
-      split_view_divider_.reset();
-  } else if (InSplitViewMode()) {
-    // If clamshell splitview mode is not enabled, fall back to the old
-    // behavior: end splitview and overivew and all windows will return to its
-    // old window state before entering tablet mode.
-    EndSplitView();
-    EndOverview();
-  }
+  EndSplitView();
 }
 
 void SplitViewController::OnTabletControllerDestroyed() {

ash/wm/splitview/split_view_controller.h

@@ -59,17 +59,18 @@ class ASH_EXPORT SplitViewController : public SplitViewNotifier,
   // top of the screen.
   enum SnapPosition { NONE, LEFT, RIGHT };
 
-  // Why splitview was ended.
+  // Why splitview was ended. For now, all reasons will be kNormal except when
+  // the home launcher button is pressed, an unsnappable window just got
+  // activated, the active user session changed, or the window dragging
+  // started.
   enum class EndReason {
     kNormal = 0,
     kHomeLauncherPressed,
     kUnsnappableWindowActivated,
     kActiveUserChanged,
     kWindowDragStarted,
-    // TODO(edcourtney): Consider not ending Split-View on PIP expand.
-    // See crbug.com/950827.
+    // TODO(950827): Consider not ending Split-View on PIP expand.
     kPipExpanded,
-    kExitTabletMode,
   };
 
   // The behaviors of split view are very different when in tablet mode and in
@@ -191,7 +192,7 @@ class ASH_EXPORT SplitViewController : public SplitViewNotifier,
                                uint32_t metrics) override;
 
   // TabletModeObserver:
-  void OnTabletModeStarting() override;
+  void OnTabletModeStarted() override;
   void OnTabletModeEnding() override;
   void OnTabletControllerDestroyed() override;
 

ash/wm/splitview/split_view_divider.cc

@@ -251,7 +251,8 @@ SplitViewDivider::~SplitViewDivider() {
   divider_widget_->Close();
   split_view_window_targeter_.reset();
   for (auto* iter : observed_windows_)
-    RemoveObservedWindow(iter);
+    iter->RemoveObserver(this);
+  observed_windows_.clear();
 }
 
 // static

ash/wm/tablet_mode/tablet_mode_controller.cc

@@ -906,8 +906,6 @@ void TabletModeController::ResetPauser() {
 }
 
 void TabletModeController::FinishInitTabletMode() {
-  for (auto& observer : tablet_mode_observers_)
-    observer.OnTabletModeStarting();
   tablet_mode_window_manager_ = std::make_unique<TabletModeWindowManager>();
   tablet_mode_window_manager_->Init();
 

ash/wm/tablet_mode/tablet_mode_observer.h

@@ -13,9 +13,6 @@ namespace ash {
 // NOTE: Code in chrome should use TabletModeClientObserver.
 class ASH_EXPORT TabletModeObserver {
  public:
-  // Called when the tablet mode is about to start.
-  virtual void OnTabletModeStarting() {}
-
   // Called when the tablet mode has started. Windows might still be animating
   // though.
   virtual void OnTabletModeStarted() {}

ash/wm/tablet_mode/tablet_mode_window_manager.h

@@ -124,10 +124,9 @@ class ASH_EXPORT TabletModeWindowManager : public aura::WindowObserver,
   void ArrangeWindowsForTabletMode();
 
   // Revert all windows to how they were arranged before tablet mode.
-  // |windows_in_splitview| contains the windows that were in splitview before
-  // entering clamshell mode.
-  void ArrangeWindowsForClamshellMode(
-      base::flat_map<aura::Window*, WindowStateType> windows_in_splitview);
+  // |was_in_overview| indicates whether it was in overview before entering
+  // desktop mode.
+  void ArrangeWindowsForDesktopMode(bool was_in_overview = false);
 
   // If the given window should be handled by us, this function will add it to
   // the list of known windows (remembering the initial show state).
@@ -138,10 +137,13 @@ class ASH_EXPORT TabletModeWindowManager : public aura::WindowObserver,
                    bool snap = false,
                    bool animate_bounds_on_attach = true);
 
-  // Remove a window from our tracking list. If the window is going to be
-  // destroyed, do not restore its old previous window state object as it will
-  // send unnecessary window state change event.
-  void ForgetWindow(aura::Window* window, bool destroyed);
+  // Remove a window from our tracking list. |was_in_overview| used when
+  // |destroyed| is false to help handle leaving tablet mode. If the window is
+  // going to be destroyed, do not restore its old previous window state object
+  // as it will send unnecessary window state change event.
+  void ForgetWindow(aura::Window* window,
+                    bool destroyed,
+                    bool was_in_overview = false);
 
   // Returns true when the given window should be modified in any way by us.
   bool ShouldHandleWindow(aura::Window* window);

ash/wm/tablet_mode/tablet_mode_window_state.cc

@@ -197,19 +197,18 @@ TabletModeWindowState::~TabletModeWindowState() {
   creator_->WindowStateDestroyed(window_);
 }
 
-void TabletModeWindowState::LeaveTabletMode(wm::WindowState* window_state) {
-  // Only do bounds change animation if the window is the top window or a window
-  // showing in splitview, and the window has changed its state. Otherwise,
-  // restore its bounds immediately.
-  EnterAnimationType animation_type =
-      window_state->IsSnapped() || IsTopWindow(window_state->window())
-          ? DEFAULT
-          : IMMEDIATE;
+void TabletModeWindowState::LeaveTabletMode(wm::WindowState* window_state,
+                                            bool was_in_overview) {
+  // TODO(minch): Keep the current animation if leaving tablet mode from
+  // overview. Need more investigation for windows' transform animation and
+  // updates bounds animation when overview is active.
+  bool use_default = was_in_overview || window_state->IsSnapped() ||
+                     IsTopWindow(window_state->window());
   if (old_state_->GetType() == window_state->GetStateType() &&
       !window_state->IsNormalStateType()) {
-    animation_type = IMMEDIATE;
+    use_default = false;
   }
-  old_state_->set_enter_animation_type(animation_type);
+  old_state_->set_enter_animation_type(use_default ? DEFAULT : IMMEDIATE);
   // Note: When we return we will destroy ourselves with the |our_reference|.
   std::unique_ptr<wm::WindowState::State> our_reference =
       window_state->SetStateObject(std::move(old_state_));

ash/wm/tablet_mode/tablet_mode_window_state.h

@@ -41,7 +41,7 @@ class TabletModeWindowState : public wm::WindowState::State {
   void set_ignore_wm_events(bool ignore) { ignore_wm_events_ = ignore; }
 
   // Leaves the tablet mode by reverting to previous state object.
-  void LeaveTabletMode(wm::WindowState* window_state);
+  void LeaveTabletMode(wm::WindowState* window_state, bool was_in_overview);
 
   // WindowState::State overrides:
   void OnWMEvent(wm::WindowState* window_state,