Cros splitscreen: Stack the dragged window below the snapped window during dragging.

Bug: 809298
Change-Id: I7d8b5e5e9a792b689129937c912bc16e6ef2e053
Reviewed-on: https://chromium-review.googlesource.com/950139Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542220}

ash/wm/overview/window_selector_item.cc

@@ -1304,6 +1304,19 @@ void WindowSelectorItem::StartDrag() {
     widget_window->parent()->StackChildAtTop(widget_window);
     widget_window->parent()->StackChildBelow(window, widget_window);
   }
+
+  // If split view mode is actvie and there is already a snapped window, stack
+  // this item's window below the snapped window. Note: this should be temporary
+  // for M66, see https://crbug.com/809298 for details.
+  if (Shell::Get()->IsSplitViewModeActive()) {
+    aura::Window* snapped_window =
+        Shell::Get()->split_view_controller()->GetDefaultSnappedWindow();
+    if (widget_window && widget_window->parent() == window->parent() &&
+        widget_window->parent() == snapped_window->parent()) {
+      widget_window->parent()->StackChildBelow(widget_window, snapped_window);
+      widget_window->parent()->StackChildBelow(window, widget_window);
+    }
+  }
 }
 
 void WindowSelectorItem::EndDrag() {