Cros: Allow scroll with mousewheel over shelf

This change will allow users to use the mousewheel to scroll over
the contents of the scrollable shelf.

Bug: 1018016
Change-Id: I949eb241a74be365b0a366f56c59cba04ce6099b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901354Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Ana Salazar <anasalazar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713271}

ash/shelf/scrollable_shelf_view.cc

@@ -1153,7 +1153,16 @@ void ScrollableShelfView::HandleMouseWheelEvent(ui::MouseWheelEvent* event) {
     return;
 
   if (GetShelf()->IsHorizontalAlignment()) {
-    ScrollByXOffset(CalculatePageScrollingOffset(event->x_offset() < 0),
+    const float x_offset = event->x_offset();
+    const float y_offset = event->y_offset();
+    // If the shelf is bottom aligned, we can scroll over the shelf contents if
+    // the scroll is horizontal or vertical (in the case of a mousewheel
+    // scroll). We take the biggest offset difference of the vertical and
+    // horizontal components to determine the offset to scroll over the
+    // contents.
+    float max_absolute_offset =
+        abs(x_offset) > abs(y_offset) ? x_offset : y_offset;
+    ScrollByXOffset(CalculatePageScrollingOffset(max_absolute_offset < 0),
                     /*animating=*/true);
   } else {
     ScrollByYOffset(CalculatePageScrollingOffset(event->y_offset() < 0),

ash/shelf/scrollable_shelf_view_unittest.cc

@@ -467,4 +467,41 @@ TEST_F(ScrollableShelfViewTest, CorrectUIInTabletWithoutOverflow) {
   EXPECT_EQ(hotseat_background.right() - 4, last_tappable_view_bounds.right());
 }
 
+// Verifies that doing a mousewheel scroll on the scrollable shelf does scroll
+// forward.
+TEST_F(ScrollableShelfViewTest, ScrollWithMouseWheel) {
+  // The scroll threshold. Taken from |KScrollOffsetThreshold| in
+  // scrollable_shelf_view.cc.
+  constexpr int scroll_threshold = 20;
+  AddAppShortcutsUntilOverflow();
+
+  ASSERT_EQ(ScrollableShelfView::kShowRightArrowButton,
+            scrollable_shelf_view_->layout_strategy_for_test());
+
+  // Do a mousewheel scroll with a positive offset bigger than the scroll
+  // threshold to scroll forward. Unlike touchpad scrolls, mousewheel scrolls
+  // can only be along the cross axis.
+  GetEventGenerator()->MoveMouseTo(
+      scrollable_shelf_view_->GetBoundsInScreen().CenterPoint());
+  GetEventGenerator()->MoveMouseWheel(0, -(scroll_threshold + 1));
+  ASSERT_EQ(ScrollableShelfView::kShowLeftArrowButton,
+            scrollable_shelf_view_->layout_strategy_for_test());
+
+  // Do a mousewheel scroll with a negative offset bigger than the scroll
+  // threshold to scroll backwards.
+  GetEventGenerator()->MoveMouseWheel(0, scroll_threshold + 1);
+  ASSERT_EQ(ScrollableShelfView::kShowRightArrowButton,
+            scrollable_shelf_view_->layout_strategy_for_test());
+
+  // Do a mousewheel scroll with an offset smaller than the scroll
+  // threshold should be ignored.
+  GetEventGenerator()->MoveMouseWheel(0, scroll_threshold);
+  ASSERT_EQ(ScrollableShelfView::kShowRightArrowButton,
+            scrollable_shelf_view_->layout_strategy_for_test());
+
+  GetEventGenerator()->MoveMouseWheel(0, -scroll_threshold);
+  ASSERT_EQ(ScrollableShelfView::kShowRightArrowButton,
+            scrollable_shelf_view_->layout_strategy_for_test());
+}
+
 }  // namespace ash