[Compositor threaded scrollbar scrolling] Mac scrollbars hit testing.

Mac doesn't |use-zoom-for-dsf|. Hence, the ScreenSpaceTransform needs
to be scaled down by the device_scale_factor for hit testing to work
as intended.

Bug: 1003576
Change-Id: I049e5d06d4ed6859e3e5a88b7fb61b46e8175801
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588948Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#702546}

cc/input/scrollbar_controller.cc

@@ -378,14 +378,21 @@ float ScrollbarController::GetScrollerToScrollbarRatio() {
   const LayerImpl* owner_scroll_layer =
       layer_tree_host_impl_->active_tree()->ScrollableLayerByElementId(
           currently_captured_scrollbar_->scroll_element_id());
-  float viewport_length =
+  const float viewport_length =
       orientation == ScrollbarOrientation::VERTICAL
           ? owner_scroll_layer->scroll_container_bounds().height()
           : (owner_scroll_layer->scroll_container_bounds().width());
+
+  // For platforms which have use_zoom_for_dsf set to false (like Mac), the
+  // device_scale_factor should not be used while determining the
+  // scaled_scroller_to_scrollbar_ratio as thumb drag would appear jittery due
+  // to constant over and under corrections.
+  // (See ScrollbarController::ScreenSpaceScaleFactor()).
   float scaled_scroller_to_scrollbar_ratio =
       ((scroll_layer_length - viewport_length) /
        (scrollbar_track_length - scrollbar_thumb_length)) *
-      layer_tree_host_impl_->active_tree()->device_scale_factor();
+      ScreenSpaceScaleFactor();
+
   return scaled_scroller_to_scrollbar_ratio;
 }
 
@@ -597,8 +604,20 @@ int ScrollbarController::GetScrollDeltaForScrollbarPart(
     default:
       scroll_delta = 0;
   }
-  return scroll_delta *
-         layer_tree_host_impl_->active_tree()->device_scale_factor();
+
+  return scroll_delta * ScreenSpaceScaleFactor();
+}
+
+float ScrollbarController::ScreenSpaceScaleFactor() const {
+  // TODO(arakeri): When crbug.com/716231 is fixed, this needs to be updated.
+  // If use_zoom_for_dsf is false, the click deltas and thumb drag ratios
+  // shouldn't be scaled. For example: On Mac, when the use_zoom_for_dsf is
+  // false and the device_scale_factor is 2, the scroll delta for pointer clicks
+  // on arrows would be incorrectly calculated as 80px instead of 40px. This is
+  // also necessary to ensure that hit testing works as intended.
+  return layer_tree_host_impl_->settings().use_zoom_for_dsf
+             ? layer_tree_host_impl_->active_tree()->device_scale_factor()
+             : 1.f;
 }
 
 gfx::PointF ScrollbarController::GetScrollbarRelativePosition(
@@ -606,7 +625,17 @@ gfx::PointF ScrollbarController::GetScrollbarRelativePosition(
     bool* clipped) {
   gfx::Transform inverse_screen_space_transform(
       gfx::Transform::kSkipInitialization);
-  if (!currently_captured_scrollbar_->ScreenSpaceTransform().GetInverse(
+
+  // If use_zoom_for_dsf is false, the ScreenSpaceTransform needs to be scaled
+  // down by the DSF to ensure that position_in_widget is transformed correctly.
+  const float scale =
+      !layer_tree_host_impl_->settings().use_zoom_for_dsf
+          ? 1.f / layer_tree_host_impl_->active_tree()->device_scale_factor()
+          : 1.f;
+  gfx::Transform scaled_screen_space_transform(
+      currently_captured_scrollbar_->ScreenSpaceTransform());
+  scaled_screen_space_transform.PostScale(scale, scale);
+  if (!scaled_screen_space_transform.GetInverse(
           &inverse_screen_space_transform))
     return gfx::PointF(0, 0);
 

cc/input/scrollbar_controller.h

@@ -74,6 +74,9 @@ class CC_EXPORT ScrollbarController {
     float scroll_position_at_start_;
   };
 
+  // Returns the DSF based on whether use-zoom-for-dsf is enabled.
+  float ScreenSpaceScaleFactor() const;
+
   // Helper to convert scroll offset to autoscroll velocity.
   float InitialDeltaToAutoscrollVelocity(gfx::ScrollOffset scroll_offset) const;
 

cc/trees/layer_tree_settings.h

@@ -129,6 +129,9 @@ class CC_EXPORT LayerTreeSettings {
   // deadlines.
   bool wait_for_all_pipeline_stages_before_draw = false;
 
+  // Determines whether the zoom needs to be applied to the device scale factor.
+  bool use_zoom_for_dsf = false;
+
   // Determines whether mouse interactions on composited scrollbars are handled
   // on the compositor thread.
   bool compositor_threaded_scrollbar_scrolling = false;

content/renderer/render_widget.cc

@@ -2917,6 +2917,7 @@ cc::LayerTreeSettings RenderWidget::GenerateLayerTreeSettings(
   const base::CommandLine& cmd = *base::CommandLine::ForCurrentProcess();
   cc::LayerTreeSettings settings;
 
+  settings.use_zoom_for_dsf = compositor_deps->IsUseZoomForDSFEnabled();
   settings.compositor_threaded_scrollbar_scrolling =
       base::FeatureList::IsEnabled(
           features::kCompositorThreadedScrollbarScrolling);

third_party/blink/web_tests/NeverFixTests

@@ -97,6 +97,8 @@ fast/harness/results.html [ WontFix ]
 [ Mac ]   fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]
 [ Linux ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]
 [ Mac ]   virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]
+[ Linux ] virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]
+[ Mac ]   virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]
 [ Linux ] virtual/scroll_customization/fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]
 [ Mac ]   virtual/scroll_customization/fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]
 [ Linux ] virtual/threaded/fast/scrolling/scrollbars/scrollbar-thumb-snapping.html [ WontFix ]

third_party/blink/web_tests/TestExpectations

@@ -3052,6 +3052,11 @@ crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/sc
 crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scroll-chaining-for-gesture-based-scrolling.html [ Failure ]
 crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-occluded-by-div.html [ Failure ]
 crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Failure Timeout ]
+crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/fast/scrolling/scrollbars/mouse-scrolling-on-div-scrollbar.html [ Failure ]
+crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/fast/scrolling/scrollbars/mouse-scrolling-on-root-scrollbar.html [ Failure ]
+crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Failure ]
+crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/fast/scrolling/scrollbars/scrollbar-occluded-by-div.html [ Failure ]
+crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Failure Timeout ]
 crbug.com/953847 [ Mac ] virtual/scroll_customization/fast/scrolling/scrollbars/mouse-scrolling-on-div-scrollbar.html [ Failure ]
 crbug.com/953847 [ Mac ] virtual/scroll_customization/fast/scrolling/scrollbars/mouse-scrolling-on-root-scrollbar.html [ Failure ]
 crbug.com/953847 [ Mac ] virtual/scroll_customization/fast/scrolling/scrollbars/scroll-chaining-for-gesture-based-scrolling.html [ Failure ]

third_party/blink/web_tests/VirtualTestSuites

@@ -529,6 +529,16 @@
              "--enable-prefer-compositing-to-lcd-text",
              "--disable-smooth-scrolling"]
   },
+  {
+    "prefix": "compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf",
+    "base": "fast/scrolling/scrollbars",
+    "args": ["--enable-features=CompositorThreadedScrollbarScrolling",
+             "--enable-threaded-compositing",
+             "--enable-prefer-compositing-to-lcd-text",
+             "--disable-smooth-scrolling",
+             "--force-device-scale-factor=2",
+             "--enable-use-zoom-for-dsf=false"]
+  },
   {
     "prefix": "compositor_threaded_scrollbar_scrolling",
     "base": "paint/invalidation/scroll/sticky",

third_party/blink/web_tests/virtual/compositor_threaded_scrollbar_scrolling_hidpi_nozoomfordsf/README.md

+This directory is dedicated for testing the "Compositor threaded
+scrollbar scrolling" feature with DSF = 2 and use_zoom_for_dsf set
+to false. This is done in order to simulate the current setup on Mac