Implement windows mousewheel scroll by page functionality

cc/trees/layer_tree_host_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_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
@@ skipping 1565 matching lines @@
 
   active_tree()->DidUpdateScroll();
   if (did_scroll) {
     client_->SetNeedsCommitOnImplThread();
     client_->SetNeedsRedrawOnImplThread();
     client_->RenewTreePriority();
   }
   return did_scroll;
 }
 
+// This implements scrolling by page as described here:
+// http://msdn.microsoft.com/en-us/library/windows/desktop/ms645601(v=vs.85).aspx#_win32_The_Mouse_Wheel
+// for events with WHEEL_PAGESCROLL set.
+bool LayerTreeHostImpl::ScrollVerticallyByPage(
+    gfx::Point viewport_point,
+    WebKit::WebScrollbar::ScrollDirection direction) {
+  DCHECK(wheel_scrolling_);
+
+  for (LayerImpl* layer_impl = CurrentlyScrollingLayer();
+       layer_impl;
+       layer_impl = layer_impl->parent()) {
+    if (!layer_impl->scrollable())
+      continue;
+
+    if (!layer_impl->vertical_scrollbar_layer())
+      continue;
+
+    float height = layer_impl->vertical_scrollbar_layer()->bounds().height();

[Vangelis Kokkevis, 2013/03/23 01:49:14]

If you're scrolling an overflow-scroll element, do you want to scroll by the
entire height of the element (which presumably is the height of its scrollbar)
or just its visible portion? 

+
+    // These magical values match WebKit and are designed to scroll nearly the
+    // entire visible content height but leave a bit of overlap.
+    float page = std::max(height * 0.875f, 1.f);
+    if (direction == WebKit::WebScrollbar::ScrollBackward)
+      page = -page;
+
+    gfx::Vector2dF delta = gfx::Vector2dF(0.f, page);
+
+    gfx::Vector2dF applied_delta = ScrollLayerWithLocalDelta(layer_impl, delta);
+
+    if (!applied_delta.IsZero()) {

[Vangelis Kokkevis, 2013/03/23 01:49:14]

scrollBy checks against a small threshold ( move_threshold_squared) to avoid
getting stuck . Seems like a good idea to do so here too.

+      active_tree()->DidUpdateScroll();
+      client_->SetNeedsCommitOnImplThread();
+      client_->SetNeedsRedrawOnImplThread();
+      client_->RenewTreePriority();
+      return true;
+    }
+

[Vangelis Kokkevis, 2013/03/23 01:49:14]

Do we care about bubbling up the remainder of the unused scroll delta? 

+    active_tree_->SetCurrentlyScrollingLayer(layer_impl);
+  }
+
+  return false;
+}
+
 void LayerTreeHostImpl::ClearCurrentlyScrollingLayer() {
   active_tree_->ClearCurrentlyScrollingLayer();
   did_lock_scrolling_layer_ = false;
 }
 
 void LayerTreeHostImpl::ScrollEnd() {
   if (top_controls_manager_)
     top_controls_manager_->ScrollEnd();
   ClearCurrentlyScrollingLayer();
   active_tree()->DidEndScroll();
@@ skipping 369 matching lines @@
   debug_state_ = debug_state;
 }
 
 void LayerTreeHostImpl::SavePaintTime(const base::TimeDelta& total_paint_time,
                                       int commit_number) {
   DCHECK(debug_state_.continuous_painting);
   paint_time_counter_->SavePaintTime(total_paint_time, commit_number);
 }
 
 }  // namespace cc