Hide scrollbars when their dimensions are not scrollable.

Source/core/frame/PinchViewport.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 49 matching lines @@
 using blink::WebScrollbarLayer;
 using blink::FrameHost;
 using blink::GraphicsLayer;
 using blink::GraphicsLayerFactory;
 
 namespace blink {
 
 PinchViewport::PinchViewport(FrameHost& owner)
     : m_frameHost(owner)
     , m_scale(1)
+    , m_hasHorizontalScrollbar(false)
+    , m_hasVerticalScrollbar(false)
 {
     reset();
 }
 
 PinchViewport::~PinchViewport() { }
 
 void PinchViewport::setSize(const IntSize& size)
 {
     if (m_size == size)
         return;
@@ skipping 72 matching lines @@
 }
 
 void PinchViewport::move(const FloatPoint& delta)
 {
     setLocation(m_offset + delta);
 }
 
 void PinchViewport::setScale(float scale)
 {
     setScaleAndLocation(scale, m_offset);
+    adjustScrollbarExistence();

[bokan, 2014/10/09 15:22:14]

Should we also call this in setSize?

 }
 
 void PinchViewport::setScaleAndLocation(float scale, const FloatPoint& location)
 {
     bool valuesChanged = false;
 
     if (scale != m_scale) {
         m_scale = scale;
         valuesChanged = true;
     }
@@ skipping 135 matching lines @@
     int yPosition = isHorizontal ? m_innerViewportContainerLayer->size().height() - scrollbarThickness : 0;
     int width = isHorizontal ? m_innerViewportContainerLayer->size().width() - scrollbarThickness : scrollbarThickness;
     int height = isHorizontal ? scrollbarThickness : m_innerViewportContainerLayer->size().height() - scrollbarThickness;
 
     // Use the GraphicsLayer to position the scrollbars.
     scrollbarGraphicsLayer->setPosition(IntPoint(xPosition, yPosition));
     scrollbarGraphicsLayer->setSize(IntSize(width, height));
     scrollbarGraphicsLayer->setContentsRect(IntRect(0, 0, width, height));
 }
 
+void PinchViewport::adjustScrollbarExistence()
+{
+    IntSize visibleSize = enclosingIntRect(visibleRect()).size();
+
+    ASSERT(mainFrame());
+    ASSERT(mainFrame()->contentRenderer());
+    IntSize documentSize = mainFrame()->contentRenderer()->documentRect().size();
+
+    bool oldHasHorizontal = m_hasHorizontalScrollbar;
+    bool oldHasVertical = m_hasVerticalScrollbar;
+    m_hasHorizontalScrollbar = visibleSize.width() < documentSize.width();
+    m_hasVerticalScrollbar = visibleSize.height() < documentSize.height();

[bokan, 2014/10/09 15:22:14]

Rather than comparing the sizes yourself here, just check if
maximumScrollPosition.x/y > 0 for this and mainFrame. There's a bunch of
subtleties to do with top controls and floating point snapping that we should
account for (coming up in a patch of mine).

+
+    if (oldHasHorizontal != m_hasHorizontalScrollbar || oldHasVertical != m_hasVerticalScrollbar) {
+        GraphicsLayerVector children;
+        children.append(m_pageScaleLayer.get());
+
+        if (m_hasHorizontalScrollbar)
+            children.append(m_overlayScrollbarHorizontal.get());
+        if (m_hasVerticalScrollbar)
+            children.append(m_overlayScrollbarVertical.get());
+
+        m_innerViewportContainerLayer->setChildren(children);

[bokan, 2014/10/09 15:22:14]

Hmm...I'm not sure what the implications of frequently adding/removing these
layers might be. Is there no "enabled" or "visible" feature on scrollbar layers?
In any case, I'll let aelias@ comment.

+    }
+}
+
 void PinchViewport::registerLayersWithTreeView(WebLayerTreeView* layerTreeView) const
 {
     TRACE_EVENT0("blink", "PinchViewport::registerLayersWithTreeView");
     ASSERT(layerTreeView);
     ASSERT(m_frameHost.page().mainFrame());
     ASSERT(m_frameHost.page().mainFrame()->isLocalFrame());
     ASSERT(m_frameHost.page().deprecatedLocalMainFrame()->contentRenderer());
 
     RenderLayerCompositor* compositor = m_frameHost.page().deprecatedLocalMainFrame()->contentRenderer()->compositor();
     // Get the outer viewport scroll layer.
@@ skipping 93 matching lines @@
     return m_innerViewportContainerLayer.get();
 }
 
 GraphicsLayer* PinchViewport::layerForScrolling() const
 {
     return m_innerViewportScrollLayer.get();
 }
 
 GraphicsLayer* PinchViewport::layerForHorizontalScrollbar() const
 {
-    return m_overlayScrollbarHorizontal.get();
+    return m_hasHorizontalScrollbar ? m_overlayScrollbarHorizontal.get() : nullptr;
 }
 
 GraphicsLayer* PinchViewport::layerForVerticalScrollbar() const
 {
-    return m_overlayScrollbarVertical.get();
+    return m_hasVerticalScrollbar ? m_overlayScrollbarVertical.get() : nullptr;
 }
 
 void PinchViewport::notifyAnimationStarted(const GraphicsLayer*, double monotonicTime)
 {
 }
 
 void PinchViewport::paintContents(const GraphicsLayer*, GraphicsContext&, GraphicsLayerPaintingPhase, const IntRect& inClip)
 {
 }
 
@@ skipping 29 matching lines @@
     } else if (graphicsLayer == m_overlayScrollbarVertical.get()) {
         name =  "Overlay Scrollbar Vertical Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink