split view: Prevent crash in SplitViewDivider::OnWindowDragStarted

Bug: 1008408
Change-Id: I1998eb54d4645741dd1f311147a0cacdb1b136c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829540
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700826}

ash/wm/overview/overview_session_unittest.cc

@@ -1299,9 +1299,7 @@ TEST_P(OverviewSessionTest, DropTargetOnCorrectDisplayForDraggingFromTop) {
 
 // Tests that dragging a window from overview creates a drop target on the same
 // display.
-// Disabled, test is flaky: crbug.com/1008408
-TEST_P(OverviewSessionTest,
-       DISABLED_DropTargetOnCorrectDisplayForDraggingFromOverview) {
+TEST_P(OverviewSessionTest, DropTargetOnCorrectDisplayForDraggingFromOverview) {
   UpdateDisplay("600x600,600x600");
   EnterTabletMode();
   // DisplayConfigurationObserver enables mirror mode when tablet mode is

ash/wm/splitview/split_view_divider.cc

@@ -343,9 +343,12 @@ void SplitViewDivider::RemoveObservedWindow(aura::Window* window) {
 void SplitViewDivider::OnWindowDragStarted(aura::Window* dragged_window) {
   is_dragging_window_ = true;
   SetAlwaysOnTop(false);
-  // Make sure |divider_widget_| is placed below the dragged window.
-  dragged_window->parent()->StackChildBelow(divider_widget_->GetNativeWindow(),
-                                            dragged_window);
+
+  aura::Window* divider_window = divider_widget_->GetNativeWindow();
+  // If |divider_window| and |dragged_window| are siblings, then make sure that
+  // |divider_window| is stacked below |dragged_window|.
+  if (divider_window->parent() == dragged_window->parent())
+    divider_window->parent()->StackChildBelow(divider_window, dragged_window);
 }
 
 void SplitViewDivider::OnWindowDragEnded() {