Hide pinch viewport scrollbars when near minimum scale.

cc/trees/layer_tree_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_impl.h"
 
 #include <limits>
 #include <set>
 
 #include "base/debug/trace_event.h"
@@ skipping 377 matching lines @@
   set_needs_update_draw_properties();
 
   if (root_layer_scroll_offset_delegate_) {
     root_layer_scroll_offset_delegate_->UpdateRootLayerState(
         TotalScrollOffset(), TotalMaxScrollOffset(), ScrollableSize(),
         current_page_scale_factor(), min_page_scale_factor_,
         max_page_scale_factor_);
   }
 
   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;

[aelias_OOO_until_Jul13, 2015/01/27 19:35:05]

I generally prefer scales to be multiplied together rather than added, adding
can have strange nonlinear effects.  Please make this a multiplication.

[bokan, 2015/01/27 20:42:01]

Done.

+    scrollbar->SetHideLayerAndSubtree(
+        current_page_scale_factor() < minimum_scale_to_show_at);
+  }
 }
 
 SyncedProperty<ScaleGroup>* LayerTreeImpl::page_scale_factor() {
   return page_scale_factor_.get();
 }
 
 const SyncedProperty<ScaleGroup>* LayerTreeImpl::page_scale_factor() const {
   return page_scale_factor_.get();
 }
 
@@ skipping 61 matching lines @@
 
   inner_viewport_scroll_layer_ =
       LayerById(inner_viewport_scroll_layer_id);
   DCHECK(inner_viewport_scroll_layer_);
 
   outer_viewport_scroll_layer_ =
       LayerById(outer_viewport_scroll_layer_id);
   DCHECK(outer_viewport_scroll_layer_ ||
          outer_viewport_scroll_layer_id == Layer::INVALID_ID);
 
+  HideInnerViewportScrollbarsIfNearMinimumScale();
+
   if (!root_layer_scroll_offset_delegate_)
     return;
 
   inner_viewport_scroll_delegate_proxy_ = make_scoped_ptr(
       new LayerScrollOffsetDelegateProxy(inner_viewport_scroll_layer_,
                                          root_layer_scroll_offset_delegate_,
                                          this));
 
   if (outer_viewport_scroll_layer_)
     outer_viewport_scroll_delegate_proxy_ = make_scoped_ptr(
@@ skipping 1034 matching lines @@
     scoped_ptr<PendingPageScaleAnimation> pending_animation) {
   pending_page_scale_animation_ = pending_animation.Pass();
 }
 
 scoped_ptr<PendingPageScaleAnimation>
     LayerTreeImpl::TakePendingPageScaleAnimation() {
   return pending_page_scale_animation_.Pass();
 }
 
 }  // namespace cc