Hide pinch viewport scrollbars when near minimum scale.

This patch prevents the overlay scrollbars from showing up when we're near the
minimum scale. The threshold can be set per-platform; by default it's 0 so the
scrollbars will be visible regardless of the scale. On Windows/Linux/CrOS we'd
like to prevent the scrollbars from overlaying over the window scrollbars so
we set a small non-0 threshold.

BUG=446411

Review URL: https://codereview.chromium.org/877863002

Cr-Commit-Position: refs/heads/master@{#313431}

cc/layers/layer_impl.cc

@@ -1347,15 +1347,8 @@ void LayerImpl::SetScrollbarPosition(ScrollbarLayerImplBase* scrollbar_layer,
     // scrolls that move the pinch virtual viewport (i.e. trigger from
     // either inner or outer viewport).
     if (scrollbar_animation_controller_) {
-      // When both non-overlay and overlay scrollbars are both present, don't
-      // animate the overlay scrollbars when page scale factor is at the min.
-      // Non-overlay scrollbars also shouldn't trigger animations.
-      bool is_animatable_scrollbar =
-          scrollbar_layer->is_overlay_scrollbar() &&
-          ((layer_tree_impl()->current_page_scale_factor() >
-            layer_tree_impl()->min_page_scale_factor()) ||
-           !layer_tree_impl()->settings().use_pinch_zoom_scrollbars);
-      if (is_animatable_scrollbar)
+      // Non-overlay scrollbars shouldn't trigger animations.
+      if (scrollbar_layer->is_overlay_scrollbar())
         scrollbar_animation_controller_->DidScrollUpdate(on_resize);
     }
   }

cc/layers/scrollbar_layer_impl_base.cc

@@ -33,8 +33,10 @@ ScrollbarLayerImplBase::~ScrollbarLayerImplBase() {}
 
 void ScrollbarLayerImplBase::PushPropertiesTo(LayerImpl* layer) {
   float active_opacity = layer->opacity();
+  bool active_hidden = layer->hide_layer_and_subtree();
   LayerImpl::PushPropertiesTo(layer);
   layer->SetOpacity(active_opacity);
+  layer->SetHideLayerAndSubtree(active_hidden);
   DCHECK(layer->ToScrollbarLayer());
   layer->ToScrollbarLayer()->set_is_overlay_scrollbar(is_overlay_scrollbar_);
   PushScrollClipPropertiesTo(layer);

cc/trees/layer_tree_impl.cc

@@ -395,6 +395,29 @@ void LayerTreeImpl::DidUpdatePageScale() {
   }
 
   ForceScrollbarParameterUpdateAfterScaleChange(page_scale_layer());
+
+  HideInnerViewportScrollbarsIfNearMinimumScale();
+}
+
+void LayerTreeImpl::HideInnerViewportScrollbarsIfNearMinimumScale() {
+  if (!InnerViewportContainerLayer())
+    return;
+
+  LayerImpl::ScrollbarSet* scrollbars =
+      InnerViewportContainerLayer()->scrollbars();
+
+  if (!scrollbars)
+    return;
+
+  for (LayerImpl::ScrollbarSet::iterator it = scrollbars->begin();
+       it != scrollbars->end();
+       ++it) {
+    ScrollbarLayerImplBase* scrollbar = *it;
+    float minimum_scale_to_show_at =
+        min_page_scale_factor() * settings().scrollbar_show_scale_threshold;
+    scrollbar->SetHideLayerAndSubtree(
+        current_page_scale_factor() < minimum_scale_to_show_at);
+  }
 }
 
 SyncedProperty<ScaleGroup>* LayerTreeImpl::page_scale_factor() {
@@ -476,6 +499,8 @@ void LayerTreeImpl::SetViewportLayersFromIds(
   DCHECK(outer_viewport_scroll_layer_ ||
          outer_viewport_scroll_layer_id == Layer::INVALID_ID);
 
+  HideInnerViewportScrollbarsIfNearMinimumScale();
+
   if (!root_layer_scroll_offset_delegate_)
     return;
 

cc/trees/layer_tree_impl.h

@@ -343,6 +343,7 @@ class CC_EXPORT LayerTreeImpl {
   bool SetPageScaleFactorLimits(float min_page_scale_factor,
                                 float max_page_scale_factor);
   void DidUpdatePageScale();
+  void HideInnerViewportScrollbarsIfNearMinimumScale();
 
   LayerTreeHostImpl* layer_tree_host_impl_;
   int source_frame_number_;

cc/trees/layer_tree_impl_unittest.cc

@@ -6,6 +6,7 @@
 
 #include "cc/layers/heads_up_display_layer_impl.h"
 #include "cc/layers/layer.h"
+#include "cc/layers/solid_color_scrollbar_layer_impl.h"
 #include "cc/test/fake_impl_proxy.h"
 #include "cc/test/fake_layer_tree_host_impl.h"
 #include "cc/test/fake_output_surface.h"
@@ -23,6 +24,7 @@ class LayerTreeImplTest : public LayerTreeHostCommonTest {
   LayerTreeImplTest() {
     LayerTreeSettings settings;
     settings.layer_transforms_should_scale_layer_contents = true;
+    settings.scrollbar_show_scale_threshold = 1.1f;
     host_impl_.reset(
         new FakeLayerTreeHostImpl(settings, &proxy_, &shared_bitmap_manager_));
     EXPECT_TRUE(host_impl_->InitializeRenderer(FakeOutputSurface::Create3d()));
@@ -1371,6 +1373,73 @@ TEST_F(LayerTreeImplTest,
   EXPECT_FALSE(result_layer);
 }
 
+TEST_F(LayerTreeImplTest, MakeScrollbarsInvisibleNearMinPageScale) {
+  const int kThumbThickness = 10;
+  const int kTrackStart = 0;
+  const bool kIsLeftSideVerticalScrollbar = false;
+  const bool kIsOverlayScrollbar = true;
+
+  LayerTreeImpl* active_tree = host_impl().active_tree();
+
+  scoped_ptr<LayerImpl> scroll_layer = LayerImpl::Create(active_tree, 1);
+  scoped_ptr<SolidColorScrollbarLayerImpl> vertical_scrollbar_layer =
+      SolidColorScrollbarLayerImpl::Create(active_tree,
+                                           2,
+                                           VERTICAL,
+                                           kThumbThickness,
+                                           kTrackStart,
+                                           kIsLeftSideVerticalScrollbar,
+                                           kIsOverlayScrollbar);
+  scoped_ptr<SolidColorScrollbarLayerImpl> horizontal_scrollbar_layer =
+      SolidColorScrollbarLayerImpl::Create(active_tree,
+                                           3,
+                                           HORIZONTAL,
+                                           kThumbThickness,
+                                           kTrackStart,
+                                           kIsLeftSideVerticalScrollbar,
+                                           kIsOverlayScrollbar);
+
+  scoped_ptr<LayerImpl> clip_layer = LayerImpl::Create(active_tree, 4);
+  scoped_ptr<LayerImpl> page_scale_layer = LayerImpl::Create(active_tree, 5);
+
+  scroll_layer->SetScrollClipLayer(clip_layer->id());
+
+  LayerImpl* scroll_layer_ptr = scroll_layer.get();
+  LayerImpl* page_scale_layer_ptr = page_scale_layer.get();
+
+  clip_layer->AddChild(page_scale_layer.Pass());
+  page_scale_layer_ptr->AddChild(scroll_layer.Pass());
+
+  vertical_scrollbar_layer->SetScrollLayerAndClipLayerByIds(
+      scroll_layer_ptr->id(),
+      clip_layer->id());
+  horizontal_scrollbar_layer->SetScrollLayerAndClipLayerByIds(
+      scroll_layer_ptr->id(),
+      clip_layer->id());
+
+  active_tree->PushPageScaleFromMainThread(1.0f, 1.0f, 4.0f);
+  active_tree->SetViewportLayersFromIds(
+      Layer::INVALID_ID,  // Overscroll
+      page_scale_layer_ptr->id(),
+      scroll_layer_ptr->id(),
+      Layer::INVALID_ID);  // Outer Scroll
+
+  EXPECT_TRUE(vertical_scrollbar_layer->hide_layer_and_subtree());
+  EXPECT_TRUE(horizontal_scrollbar_layer->hide_layer_and_subtree());
+
+  active_tree->PushPageScaleFromMainThread(1.05f, 1.0f, 4.0f);
+  EXPECT_TRUE(vertical_scrollbar_layer->hide_layer_and_subtree());
+  EXPECT_TRUE(horizontal_scrollbar_layer->hide_layer_and_subtree());
+
+  active_tree->PushPageScaleFromMainThread(1.1f, 1.0f, 4.0f);
+  EXPECT_FALSE(vertical_scrollbar_layer->hide_layer_and_subtree());
+  EXPECT_FALSE(horizontal_scrollbar_layer->hide_layer_and_subtree());
+
+  active_tree->PushPageScaleFromMainThread(1.5f, 1.0f, 4.0f);
+  EXPECT_FALSE(vertical_scrollbar_layer->hide_layer_and_subtree());
+  EXPECT_FALSE(horizontal_scrollbar_layer->hide_layer_and_subtree());
+}
+
 TEST_F(LayerTreeImplTest,
        HitCheckingTouchHandlerRegionsForSinglePositionedLayer) {
   scoped_ptr<LayerImpl> root =

cc/trees/layer_tree_settings.cc

@@ -35,6 +35,7 @@ LayerTreeSettings::LayerTreeSettings()
       scrollbar_fade_delay_ms(0),
       scrollbar_fade_resize_delay_ms(0),
       scrollbar_fade_duration_ms(0),
+      scrollbar_show_scale_threshold(1.0f),
       solid_color_scrollbar_color(SK_ColorWHITE),
       calculate_top_controls_position(false),
       timeout_and_draw_when_animation_checkerboards(true),

cc/trees/layer_tree_settings.h

@@ -47,6 +47,7 @@ class CC_EXPORT LayerTreeSettings {
   int scrollbar_fade_delay_ms;
   int scrollbar_fade_resize_delay_ms;
   int scrollbar_fade_duration_ms;
+  float scrollbar_show_scale_threshold;
   SkColor solid_color_scrollbar_color;
   bool calculate_top_controls_position;
   bool timeout_and_draw_when_animation_checkerboards;

content/renderer/gpu/render_widget_compositor.cc

@@ -403,6 +403,11 @@ void RenderWidgetCompositor::Initialize() {
   settings.scrollbar_fade_delay_ms = 500;
   settings.scrollbar_fade_resize_delay_ms = 500;
   settings.scrollbar_fade_duration_ms = 300;
+
+  // When pinching in, only show the pinch-viewport overlay scrollbars if the
+  // page scale is at least some threshold away from the minimum. i.e. don't
+  // show the pinch scrollbars when at minimum scale.
+  settings.scrollbar_show_scale_threshold = 1.05f;
 #endif
 
   if (cmd->HasSwitch(switches::kEnableLowResTiling))