split view: Fix case with one tablet split view window and no overview

See the videos linked in https://crbug.com/1027179.

Fixed: 1027179
Change-Id: Ia4f4e112b02297f3d43da81935ee1e97215c3d85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1979146Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729997}

ash/wm/overview/overview_session.h

@@ -82,7 +82,10 @@ class ASH_EXPORT OverviewSession : public display::DisplayObserver,
     kSwipeFromShelf,
     // Used only when it's desired to enter overview mode immediately without
     // animations. It's used when entering overview by dragging a window from
-    // from the top of the screen.
+    // the top of the screen or from the shelf. It's also used when entering
+    // overview to avoid the blatantly broken behaviors shown in the videos
+    // linked in https://crbug.com/1027179. This should not be used for exiting
+    // overview mode.
     kImmediateEnter,
     // Used only when it's desired to exit overview mode immediately without
     // animations. This is used when performing the desk switch animation when

ash/wm/splitview/split_view_controller.cc

@@ -1124,6 +1124,21 @@ void SplitViewController::OnOverviewModeEnding(
   ShowAppCannotSnapToast();
 }
 
+void SplitViewController::OnDisplayRemoved(
+    const display::Display& old_display) {
+  // Display removal always triggers a window activation which ends overview,
+  // and therefore ends clamshell split view, before |OnDisplayRemoved| is
+  // called. In clamshell mode, |OverviewController::CanEndOverview| always
+  // returns true, meaning that overview is guaranteed to end successfully.
+  DCHECK(!InClamshellSplitViewMode());
+  // If we are in tablet split view with only one snapped window, make sure we
+  // are in overview (see https://crbug.com/1027179).
+  if (state_ == State::kLeftSnapped || state_ == State::kRightSnapped) {
+    Shell::Get()->overview_controller()->StartOverview(
+        OverviewSession::EnterExitOverviewType::kImmediateEnter);
+  }
+}
+
 void SplitViewController::OnDisplayMetricsChanged(
     const display::Display& display,
     uint32_t metrics) {

ash/wm/splitview/split_view_controller.h

@@ -236,6 +236,7 @@ class ASH_EXPORT SplitViewController : public aura::WindowObserver,
   void OnOverviewModeEnding(OverviewSession* overview_session) override;
 
   // display::DisplayObserver:
+  void OnDisplayRemoved(const display::Display& old_display) override;
   void OnDisplayMetricsChanged(const display::Display& display,
                                uint32_t metrics) override;
 

ash/wm/tablet_mode/tablet_mode_controller_unittest.cc

@@ -1451,6 +1451,25 @@ TEST_P(
   base::RunLoop().RunUntilIdle();
 }
 
+TEST_P(TabletModeControllerTest,
+       StartTabletActiveLeftSnapOnPrimaryDisplayPreviousOnSecondaryDisplay) {
+  UpdateDisplay("800x600,800x600");
+  std::unique_ptr<aura::Window> window1 =
+      CreateDesktopWindowSnappedLeft(gfx::Rect(0, 0, 400, 400));
+  EXPECT_EQ(Shell::GetPrimaryRootWindow(), window1->GetRootWindow());
+  std::unique_ptr<aura::Window> window2 =
+      CreateTestWindow(gfx::Rect(800, 0, 400, 400));
+  EXPECT_NE(Shell::GetPrimaryRootWindow(), window2->GetRootWindow());
+  wm::ActivateWindow(window1.get());
+  tablet_mode_controller()->SetEnabledForTest(true);
+  // After display mirroring triggers, as the split view state will still be
+  // |SplitViewController::State::kLeftSnapped|, check for overview mode.
+  base::RunLoop().RunUntilIdle();
+  EXPECT_EQ(SplitViewController::State::kLeftSnapped,
+            split_view_controller()->state());
+  EXPECT_TRUE(Shell::Get()->overview_controller()->InOverviewSession());
+}
+
 // Test that tablet mode controller does not respond to the input device changes
 // during its suspend.
 TEST_P(TabletModeControllerTest, DoNotObserverInputDeviceChangeDuringSuspend) {