Hide scrollbars when their dimensions are not scrollable.

Source/core/frame/PinchViewport.cpp

Index: Source/core/frame/PinchViewport.cpp
diff --git a/Source/core/frame/PinchViewport.cpp b/Source/core/frame/PinchViewport.cpp
index 3c877332714d5e26b104395b96ea3ed1bfde8e5f..4129038aaab26cd72ad21f27d1e5d0a8168bcee9 100644
--- a/Source/core/frame/PinchViewport.cpp
+++ b/Source/core/frame/PinchViewport.cpp
@@ -67,6 +67,8 @@ namespace blink {
 PinchViewport::PinchViewport(FrameHost& owner)
     : m_frameHost(owner)
     , m_scale(1)
+    , m_hasHorizontalScrollbar(false)
+    , m_hasVerticalScrollbar(false)
 {
     reset();
 }
@@ -159,6 +161,7 @@ void PinchViewport::move(const FloatPoint& 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)
@@ -314,6 +317,32 @@ void PinchViewport::setupScrollbar(WebScrollbar::Orientation orientation)
     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");
@@ -427,12 +456,12 @@ GraphicsLayer* PinchViewport::layerForScrolling() const
 
 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)