overview gesture: the dragged window can't exceed the top of the display.

Bug: 997885
Change-Id: I1e7df1158e2d1751339c54e0758b4d2d45172f5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888396Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710812}

ash/home_screen/drag_window_from_shelf_controller.cc

@@ -29,6 +29,7 @@
 #include "ui/compositor/layer_animation_observer.h"
 #include "ui/compositor/scoped_layer_animation_settings.h"
 #include "ui/display/screen.h"
+#include "ui/gfx/transform_util.h"
 #include "ui/wm/core/coordinate_conversion.h"
 
 namespace ash {
@@ -452,6 +453,22 @@ void DragWindowFromShelfController::UpdateDraggedWindow(
           (initial_location_in_screen_.y() - bounds.y()) * scale);
   transform.Scale(scale, scale);
 
+  // The dragged window cannot exceed the top of the display. So calculate the
+  // expected transformed bounds and then adjust the transform if needed.
+  gfx::RectF transformed_bounds(window_->bounds());
+  gfx::Transform new_tranform = TransformAboutPivot(
+      gfx::ToRoundedPoint(transformed_bounds.origin()), transform);
+  new_tranform.TransformRect(&transformed_bounds);
+  ::wm::TranslateRectToScreen(window_->parent(), &transformed_bounds);
+  const gfx::Rect display_bounds =
+      display::Screen::GetScreen()
+          ->GetDisplayNearestPoint(location_in_screen)
+          .bounds();
+  if (transformed_bounds.y() < display_bounds.y()) {
+    transform.Translate(0,
+                        (display_bounds.y() - transformed_bounds.y()) / scale);
+  }
+
   SetTransform(window_, transform);
 }