Clip scrollbars to box bounds when they don't fit.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index d61ec02d198ed490b539551d63177555d9def36d..21c7ec14add5521c44300d1a72d71d5f64b70304 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -709,11 +709,11 @@ void CompositedLayerMapping::updateGraphicsLayerGeometry(const PaintLayer* compo
 
     // Might update graphicsLayerParentLocation.
     updateAncestorClippingLayerGeometry(compositingContainer, snappedOffsetFromCompositedAncestor, graphicsLayerParentLocation);
-    updateOverflowControlsHostLayerGeometry(compositingStackingContext, compositingContainer);
 
     FloatSize contentsSize = relativeCompositingBounds.size();
 
     updateMainGraphicsLayerGeometry(relativeCompositingBounds, localCompositingBounds, graphicsLayerParentLocation);
+    updateOverflowControlsHostLayerGeometry(compositingStackingContext, compositingContainer);
     updateContentsOffsetInCompositingLayer(snappedOffsetFromCompositedAncestor, graphicsLayerParentLocation);
     updateSquashingLayerGeometry(offsetFromCompositedAncestor, graphicsLayerParentLocation, m_owningLayer, m_squashedLayers, m_squashingLayer.get(), &m_squashingLayerOffsetFromTransformedAncestor, layersNeedingPaintInvalidation);
 
@@ -828,12 +828,14 @@ void CompositedLayerMapping::updateOverflowControlsHostLayerGeometry(const Paint
     if (!m_overflowControlsHostLayer)
         return;
 
+    LayoutPoint hostLayerPosition;
+
     if (needsToReparentOverflowControls()) {
-        if (m_overflowControlsClippingLayer) {
-            m_overflowControlsClippingLayer->setSize(m_ancestorClippingLayer->size());
-            m_overflowControlsClippingLayer->setOffsetFromLayoutObject(m_ancestorClippingLayer->offsetFromLayoutObject());
-            m_overflowControlsClippingLayer->setMasksToBounds(true);
-            m_overflowControlsHostLayer->setPosition(IntPoint(-m_overflowControlsClippingLayer->offsetFromLayoutObject()));
+        if (m_overflowControlsAncestorClippingLayer) {
+            m_overflowControlsAncestorClippingLayer->setSize(m_ancestorClippingLayer->size());
+            m_overflowControlsAncestorClippingLayer->setOffsetFromLayoutObject(m_ancestorClippingLayer->offsetFromLayoutObject());
+            m_overflowControlsAncestorClippingLayer->setMasksToBounds(true);
+            hostLayerPosition = toLayoutPoint(LayoutSize(-m_overflowControlsAncestorClippingLayer->offsetFromLayoutObject()));
 
             FloatPoint position = m_ancestorClippingLayer->position();
             if (compositingStackingContext != compositingContainer) {
@@ -843,20 +845,25 @@ void CompositedLayerMapping::updateOverflowControlsHostLayerGeometry(const Paint
                 position += offsetFromStackingContainer;
             }
 
-            m_overflowControlsClippingLayer->setPosition(position);
+            m_overflowControlsAncestorClippingLayer->setPosition(position);
         } else {
             // The controls are in the same 2D space as the compositing container, so we can map them into the space of the container.
             TransformState transformState(TransformState::ApplyTransformDirection, FloatPoint());
             m_owningLayer.layoutObject()->mapLocalToContainer(compositingStackingContext->layoutObject(), transformState, ApplyContainerFlip);
             transformState.flatten();
-            LayoutPoint offsetFromStackingContainer = LayoutPoint(transformState.lastPlanarPoint());
+            hostLayerPosition = LayoutPoint(transformState.lastPlanarPoint());
             if (PaintLayerScrollableArea* scrollableArea = compositingStackingContext->scrollableArea())
-                offsetFromStackingContainer.move(LayoutSize(scrollableArea->adjustedScrollOffset()));
-            m_overflowControlsHostLayer->setPosition(FloatPoint(offsetFromStackingContainer));
+                hostLayerPosition.move(LayoutSize(scrollableArea->adjustedScrollOffset()));
         }
-    } else {
-        m_overflowControlsHostLayer->setPosition(FloatPoint());
     }
+
+    IntSize offsetFromLayoutObject = m_graphicsLayer->offsetFromLayoutObject() - roundedIntSize(m_owningLayer.subpixelAccumulation());

[chrishtr, 2015/11/19 17:29:52]

Add comments explaining what this is doing and why this is the correct formula.

[skobes, 2015/11/19 18:30:07]

Done.

+    hostLayerPosition.move(-offsetFromLayoutObject);
+    m_overflowControlsHostLayer->setPosition(FloatPoint(hostLayerPosition));
+
+    const IntRect borderBox = toLayoutBox(m_owningLayer.layoutObject())->pixelSnappedBorderBoxRect();
+    m_overflowControlsHostLayer->setSize(borderBox.size());
+    m_overflowControlsHostLayer->setMasksToBounds(true);
 }
 
 void CompositedLayerMapping::updateChildContainmentLayerGeometry(const IntRect& clippingBox, const IntRect& localCompositingBounds)
@@ -1103,7 +1110,7 @@ void CompositedLayerMapping::updateInternalHierarchy()
     bottomLayer = m_graphicsLayer.get();
     if (m_isMainFrameLayoutViewLayer)
         bottomLayer = layoutObject()->frame()->page()->frameHost().visualViewport().containerLayer();
-    updateBottomLayer(m_overflowControlsClippingLayer.get());
+    updateBottomLayer(m_overflowControlsAncestorClippingLayer.get());
     updateBottomLayer(m_overflowControlsHostLayer.get());
     if (m_layerForHorizontalScrollbar)
         m_overflowControlsHostLayer->addChild(m_layerForHorizontalScrollbar.get());
@@ -1344,8 +1351,8 @@ bool CompositedLayerMapping::updateOverflowControlsLayers(bool needsHorizontalSc
 
     bool needsOverflowControlsHostLayer = needsHorizontalScrollbarLayer || needsVerticalScrollbarLayer || needsScrollCornerLayer;
     toggleScrollbarLayerIfNeeded(m_overflowControlsHostLayer, needsOverflowControlsHostLayer, CompositingReasonLayerForOverflowControlsHost);
-    bool needsOverflowClipLayer = needsOverflowControlsHostLayer && needsAncestorClip;
-    toggleScrollbarLayerIfNeeded(m_overflowControlsClippingLayer, needsOverflowClipLayer, CompositingReasonLayerForOverflowControlsHost);
+    bool needsOverflowAncestorClipLayer = needsOverflowControlsHostLayer && needsAncestorClip;
+    toggleScrollbarLayerIfNeeded(m_overflowControlsAncestorClippingLayer, needsOverflowAncestorClipLayer, CompositingReasonLayerForOverflowControlsHost);
 
     if (ScrollingCoordinator* scrollingCoordinator = scrollingCoordinatorFromLayer(m_owningLayer)) {
         if (horizontalScrollbarLayerChanged)
@@ -1359,11 +1366,10 @@ bool CompositedLayerMapping::updateOverflowControlsLayers(bool needsHorizontalSc
 
 void CompositedLayerMapping::positionOverflowControlsLayers()
 {
-    IntSize offsetFromLayoutObject = m_graphicsLayer->offsetFromLayoutObject() - roundedIntSize(m_owningLayer.subpixelAccumulation());
     if (GraphicsLayer* layer = layerForHorizontalScrollbar()) {
         Scrollbar* hBar = m_owningLayer.scrollableArea()->horizontalScrollbar();
         if (hBar) {
-            layer->setPosition(hBar->frameRect().location() - offsetFromLayoutObject);

[chrishtr, 2015/11/19 17:29:52]

subpixelAccumulation was incorrectly omitted before?

[skobes, 2015/11/19 18:30:07]

No, what I have done is to move this offset out of the individual scrollbar
layers and into the positioning of their parent in
updateOverflowControlsHostLayerGeometry.  There should be no change in behavior
with respect to subpixel accumulation.

Though I admit that I do not really understand the role of
PaintLayer::subpixelAccumulation() in this computation.

+            layer->setPosition(hBar->frameRect().location());
             layer->setSize(hBar->frameRect().size());
             if (layer->hasContentsLayer())
                 layer->setContentsRect(IntRect(IntPoint(), hBar->frameRect().size()));
@@ -1374,7 +1380,7 @@ void CompositedLayerMapping::positionOverflowControlsLayers()
     if (GraphicsLayer* layer = layerForVerticalScrollbar()) {
         Scrollbar* vBar = m_owningLayer.scrollableArea()->verticalScrollbar();
         if (vBar) {
-            layer->setPosition(vBar->frameRect().location() - offsetFromLayoutObject);
+            layer->setPosition(vBar->frameRect().location());
             layer->setSize(vBar->frameRect().size());
             if (layer->hasContentsLayer())
                 layer->setContentsRect(IntRect(IntPoint(), vBar->frameRect().size()));
@@ -1384,7 +1390,7 @@ void CompositedLayerMapping::positionOverflowControlsLayers()
 
     if (GraphicsLayer* layer = layerForScrollCorner()) {
         const IntRect& scrollCornerAndResizer = m_owningLayer.scrollableArea()->scrollCornerAndResizerRect();
-        layer->setPosition(FloatPoint(scrollCornerAndResizer.location() - offsetFromLayoutObject));
+        layer->setPosition(FloatPoint(scrollCornerAndResizer.location()));
         layer->setSize(FloatSize(scrollCornerAndResizer.size()));
         layer->setDrawsContent(!scrollCornerAndResizer.isEmpty());
     }
@@ -1959,7 +1965,7 @@ bool CompositedLayerMapping::needsToReparentOverflowControls() const
 
 GraphicsLayer* CompositedLayerMapping::detachLayerForOverflowControls(const PaintLayer& enclosingLayer)
 {
-    GraphicsLayer* host = m_overflowControlsClippingLayer.get();
+    GraphicsLayer* host = m_overflowControlsAncestorClippingLayer.get();
     if (!host)
         host = m_overflowControlsHostLayer.get();
     host->removeFromParent();
@@ -1983,6 +1989,22 @@ GraphicsLayer* CompositedLayerMapping::parentForSublayers() const
     return m_graphicsLayer.get();
 }
 
+void CompositedLayerMapping::setSublayers(const GraphicsLayerVector& sublayers)
+{
+    GraphicsLayer* overflowControlsContainer = m_overflowControlsAncestorClippingLayer
+        ? m_overflowControlsAncestorClippingLayer.get()
+        : m_overflowControlsHostLayer.get();
+    GraphicsLayer* parent = parentForSublayers();
+    bool needsOverflowControlsReattached = overflowControlsContainer && overflowControlsContainer->parent() == parent;
+
+    parent->setChildren(sublayers);
+
+    // If we have scrollbars, but are not using composited scrolling, then parentForSublayers may return m_graphicsLayer.
+    // In that case, the above call to setChildren has clobbered the overflow controls host layer, so we need to reattach it.
+    if (needsOverflowControlsReattached)
+        parent->addChild(overflowControlsContainer);
+}
+
 GraphicsLayer* CompositedLayerMapping::childForSuperlayers() const
 {
     if (m_squashingContainmentLayer)
@@ -2502,8 +2524,8 @@ String CompositedLayerMapping::debugName(const GraphicsLayer* graphicsLayer) con
         name = "Scroll Corner Layer";
     } else if (graphicsLayer == m_overflowControlsHostLayer.get()) {
         name = "Overflow Controls Host Layer";
-    } else if (graphicsLayer == m_overflowControlsClippingLayer.get()) {
-        name = "Overflow Controls ClipLayer Layer";
+    } else if (graphicsLayer == m_overflowControlsAncestorClippingLayer.get()) {
+        name = "Overflow Controls Ancestor Clipping Layer";
     } else if (graphicsLayer == m_scrollingLayer.get()) {
         name = "Scrolling Layer";
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {