Allow perfect scroll bubbling between outer/inner viewports

cc/trees/layer_tree_host_impl_unittest.cc

Index: cc/trees/layer_tree_host_impl_unittest.cc
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index e128013c25ebe18f5e19259a9be646dc5abd041c..cf6213d5c30ec70284c100682e5fe0fb4527b313 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -7479,6 +7479,49 @@ TEST_F(LayerTreeHostImplVirtualViewportTest, FlingScrollBubblesToInner) {
   }
 }
 
+TEST_F(LayerTreeHostImplVirtualViewportTest,
+       DiagonalScrollBubblesPerfectlyToInner) {
+  gfx::Size content_size = gfx::Size(100, 160);
+  gfx::Size outer_viewport = gfx::Size(50, 80);
+  gfx::Size inner_viewport = gfx::Size(25, 40);
+
+  SetupVirtualViewportLayers(content_size, outer_viewport, inner_viewport);
+
+  LayerImpl* outer_scroll = host_impl_->OuterViewportScrollLayer();
+  LayerImpl* inner_scroll = host_impl_->InnerViewportScrollLayer();
+  DrawFrame();
+  {
+    gfx::Vector2dF inner_expected;
+    gfx::Vector2dF outer_expected;
+    EXPECT_VECTOR_EQ(inner_expected, inner_scroll->TotalScrollOffset());
+    EXPECT_VECTOR_EQ(outer_expected, outer_scroll->TotalScrollOffset());
+
+    // Make sure the scroll goes to the outer viewport first.
+    EXPECT_EQ(InputHandler::ScrollStarted,
+              host_impl_->ScrollBegin(gfx::Point(), InputHandler::Gesture));
+    EXPECT_EQ(InputHandler::ScrollStarted, host_impl_->FlingScrollBegin());
+
+    // Scroll near the edge of the outer viewport.
+    gfx::Vector2d scroll_delta(inner_viewport.width(), inner_viewport.height());
+    host_impl_->ScrollBy(gfx::Point(), scroll_delta);
+    outer_expected += scroll_delta;
+
+    EXPECT_VECTOR_EQ(inner_expected, inner_scroll->TotalScrollOffset());
+    EXPECT_VECTOR_EQ(outer_expected, outer_scroll->TotalScrollOffset());
+
+    // Now diagonal scroll across the outer viewport boundary in a single event.
+    // The entirety of the scroll should be consumed, as bubbling between inner
+    // and outer viewport layers is perfect.
+    host_impl_->ScrollBy(gfx::Point(), gfx::ScaleVector2d(scroll_delta, 2));
+    outer_expected += scroll_delta;
+    inner_expected += scroll_delta;
+    host_impl_->ScrollEnd();
+
+    EXPECT_VECTOR_EQ(inner_expected, inner_scroll->TotalScrollOffset());
+    EXPECT_VECTOR_EQ(outer_expected, outer_scroll->TotalScrollOffset());
+  }
+}
+
 class LayerTreeHostImplWithImplicitLimitsTest : public LayerTreeHostImplTest {
  public:
   void SetUp() override {