Remove arbitrary fractional rounding in viewport scroll.

This rounding was arbitrary but previous naive attempts to remove it resulted
in top control jittering so it was left in. With the viewport scroll order
change in r348988, this caused fractional scrolls to be dropped completely.
Removing it now though doesn't seem to cause the top control problem.

BUG=539334
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1387333003

Cr-Commit-Position: refs/heads/master@{#352945}

2 files changed, 53 insertions, 11 deletions

diff --git a/cc/layers/viewport.cc b/cc/layers/viewport.cc
index 80e711f..4b68de5 100644
--- a/cc/layers/viewport.cc
+++ b/cc/layers/viewport.cc
@@ -50,17 +50,11 @@ Viewport::ScrollResult Viewport::ScrollBy(const gfx::Vector2dF& delta,
 
   ScrollResult result;
 
-  // TODO(bokan): This shouldn't be needed but removing it causes subtle
-  // viewport movement during top controls manipulation.
-  if (gfx::ToRoundedVector2d(pending_content_delta).IsZero()) {
-    result.consumed_delta = delta;
-  } else {
-    pending_content_delta -= host_impl_->ScrollLayer(OuterScrollLayer(),
-                                                     pending_content_delta,
-                                                     viewport_point,
-                                                     is_direct_manipulation);
-    result.consumed_delta = delta - AdjustOverscroll(pending_content_delta);
-  }
+  pending_content_delta -= host_impl_->ScrollLayer(OuterScrollLayer(),
+                                                   pending_content_delta,
+                                                   viewport_point,
+                                                   is_direct_manipulation);
+  result.consumed_delta = delta - AdjustOverscroll(pending_content_delta);
 
   result.content_scrolled_delta = content_delta - pending_content_delta;
   return result;
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index 0de56bf..8f737ee 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -1311,6 +1311,54 @@ TEST_F(LayerTreeHostImplTest, ViewportScrollOrder) {
       outer_scroll_layer->CurrentScrollOffset());
 }
 
+// Make sure scrolls smaller than a unit applied to the viewport don't get
+// dropped. crbug.com/539334.
+TEST_F(LayerTreeHostImplTest, ScrollViewportWithFractionalAmounts) {
+  LayerTreeSettings settings = DefaultSettings();
+  CreateHostImpl(settings, CreateOutputSurface());
+  host_impl_->active_tree()->PushPageScaleFromMainThread(1.f, 1.f, 2.f);
+
+  const gfx::Size content_size(1000, 1000);
+  const gfx::Size viewport_size(500, 500);
+  CreateBasicVirtualViewportLayers(viewport_size, content_size);
+
+  LayerImpl* outer_scroll_layer = host_impl_->OuterViewportScrollLayer();
+  LayerImpl* inner_scroll_layer = host_impl_->InnerViewportScrollLayer();
+
+  // Sanity checks.
+  EXPECT_VECTOR_EQ(
+      gfx::Vector2dF(500, 500),
+      outer_scroll_layer->MaxScrollOffset());
+  EXPECT_VECTOR_EQ(gfx::Vector2dF(), outer_scroll_layer->CurrentScrollOffset());
+  EXPECT_VECTOR_EQ(gfx::Vector2dF(), inner_scroll_layer->CurrentScrollOffset());
+
+  RebuildPropertyTrees();
+
+  // Scroll only the layout viewport.
+  host_impl_->ScrollBegin(gfx::Point(250, 250), InputHandler::GESTURE);
+  host_impl_->ScrollBy(gfx::Point(250, 250), gfx::Vector2dF(0.125f, 0.125f));
+  EXPECT_VECTOR2DF_EQ(
+      gfx::Vector2dF(0.125f, 0.125f),
+      outer_scroll_layer->CurrentScrollOffset());
+  EXPECT_VECTOR2DF_EQ(
+      gfx::Vector2dF(0, 0),
+      inner_scroll_layer->CurrentScrollOffset());
+  host_impl_->ScrollEnd();
+
+  host_impl_->active_tree()->PushPageScaleFromMainThread(2.f, 1.f, 2.f);
+
+  // Now that we zoomed in, the scroll should be applied to the inner viewport.
+  host_impl_->ScrollBegin(gfx::Point(250, 250), InputHandler::GESTURE);
+  host_impl_->ScrollBy(gfx::Point(250, 250), gfx::Vector2dF(0.5f, 0.5f));
+  EXPECT_VECTOR2DF_EQ(
+      gfx::Vector2dF(0.125f, 0.125f),
+      outer_scroll_layer->CurrentScrollOffset());
+  EXPECT_VECTOR2DF_EQ(
+      gfx::Vector2dF(0.25f, 0.25f),
+      inner_scroll_layer->CurrentScrollOffset());
+  host_impl_->ScrollEnd();
+}
+
 // Tests that scrolls during a pinch gesture (i.e. "two-finger" scrolls) work
 // as expected. That is, scrolling during a pinch should bubble from the inner
 // to the outer viewport.