Refactor how we set up the layers hierarchy when using fixed

composited layers, and fix the z-index position.

Bug:2497910 Bug:2450006

We add a new COMPOSITED_FIXED_ELEMENTS define to isolate the
changes in the webkit common code.

We previously had a problem where the hierarchy of GraphicsLayer
(i.e. the backed surfaces associated to the composited RenderLayer)
was not reflecting that layers were children of a fixed layer.
The workaround we currently have is not fully satisfactory, due to the way
we draw layers on screen (in some cases layers were wrongly translated,
see Bug:2497910).

Instead, modifying the webkit common code simplify things a lot, and makes
the patch more likely to be upstreamed to webkit, as it's now a
reasonably well-delimited feature (use composited layers for fixed elements).

What we do now is to consider fixed elements as a stacking context, which
makes all layers children of such elements children too in the
GraphicsLayer hierarchy, and modifying the offset of those children
accordingly (in RenderLayer.cpp).

In addition, we fixes the z-index bugs we had
by signaling that there is a fixed element to its siblings, and turning
the siblings as composited layers as well (so that the ordering works
fully UI-side).

Change-Id: I735c6c14d955ef54653f0053187d3495bef1f332

4 files changed, 3 insertions, 102 deletions

diff --git a/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp b/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp
index b48ef49..93469b0 100644
--- a/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp
+++ b/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp
@@ -218,39 +218,6 @@ void GraphicsLayerAndroid::needsSyncChildren()
     askForSync();
 }
 
-void GraphicsLayerAndroid::syncFixedDescendants()
-{
-    for (unsigned int i = 0; i < m_children.size(); i++)
-        (static_cast<GraphicsLayerAndroid*>(m_children[i]))->syncFixedDescendants();
-
-    if (!m_client)
-        return;
-
-    RenderLayerBacking* backing = static_cast<RenderLayerBacking*>(m_client);
-    RenderLayer* renderLayer = backing->owningLayer();
-    RenderView* view = static_cast<RenderView*>(renderLayer->renderer());
-
-    // If we have an ancestor that is a fixed position layer, we need to
-    // mark ourselve as a child of it, as the RenderLayer hierarchy only
-    // keeps track of the z-order.
-    // By calling setRelativeTo() we ensure that we will keep track of
-    // the fixed layer we are relative to, and will be able to update
-    // our position accordingly.
-    RenderLayer* positionedParent = renderLayer->parent();
-    while (positionedParent &&
-           !(positionedParent->renderer()->isPositioned() &&
-           positionedParent->renderer()->style()->position() == FixedPosition))
-        positionedParent = positionedParent->parent();
-
-    if (positionedParent && positionedParent->isComposited()) {
-        RenderLayerBacking* positionedParentBacking = positionedParent->backing();
-        GraphicsLayerAndroid* positionedParentLayer =
-            static_cast<GraphicsLayerAndroid*>(positionedParentBacking->graphicsLayer());
-        LayerAndroid* parentLayer = positionedParentLayer->contentLayer();
-        m_contentLayer->setRelativeTo(parentLayer);
-    }
-}
-
 void GraphicsLayerAndroid::updateFixedPosition()
 {
     if (!m_client)
@@ -456,7 +423,6 @@ void GraphicsLayerAndroid::sendImmediateRepaint()
     if (rootGraphicsLayer->m_frame
         && rootGraphicsLayer->m_frame->view()) {
         LayerAndroid* copyLayer = new LayerAndroid(*m_contentLayer);
-        copyLayer->ensureFixedLayersForDescendants(copyLayer);
         TLOG("(%x) sendImmediateRepaint, copy the layer, (%.2f,%.2f => %.2f,%.2f)",
             this, m_contentLayer->getSize().width(), m_contentLayer->getSize().height(),
             copyLayer->getSize().width(), copyLayer->getSize().height());
@@ -920,7 +886,6 @@ void GraphicsLayerAndroid::syncCompositingState()
     syncChildren();
     syncMask();
     syncPositionState();
-    syncFixedDescendants();
 
     if (!gPaused || WTF::currentTime() >= gPausedDelay)
         repaintAll();
diff --git a/WebCore/platform/graphics/android/GraphicsLayerAndroid.h b/WebCore/platform/graphics/android/GraphicsLayerAndroid.h
index 09f4181..25f70b4 100644
--- a/WebCore/platform/graphics/android/GraphicsLayerAndroid.h
+++ b/WebCore/platform/graphics/android/GraphicsLayerAndroid.h
@@ -110,7 +110,6 @@ public:
     virtual void setZPosition(float);
 
     void askForSync();
-    void syncFixedDescendants();
     void syncPositionState();
     void needsSyncChildren();
     void syncChildren();
diff --git a/WebCore/platform/graphics/android/LayerAndroid.cpp b/WebCore/platform/graphics/android/LayerAndroid.cpp
index 26372f0..c17a034 100644
--- a/WebCore/platform/graphics/android/LayerAndroid.cpp
+++ b/WebCore/platform/graphics/android/LayerAndroid.cpp
@@ -49,14 +49,10 @@ LayerAndroid::LayerAndroid(bool isRootLayer) : SkLayer(),
     m_haveClip(false),
     m_doRotation(false),
     m_isFixed(false),
-    m_isRelativeTo(false),
-    m_relativeFixedLayerID(0),
     m_recordingPicture(0),
     m_extra(0),
-    m_uniqueId(++gUniqueId),
-    m_relativeFixedLayer(0)
+    m_uniqueId(++gUniqueId)
 {
-    m_deltaPosition.set(0, 0);
     m_angleTransform = 0;
     m_translation.set(0, 0);
     m_scale.set(1, 1);
@@ -69,14 +65,10 @@ LayerAndroid::LayerAndroid(const LayerAndroid& layer) : SkLayer(layer),
     m_isRootLayer(layer.m_isRootLayer),
     m_haveClip(layer.m_haveClip),
     m_extra(0), // deliberately not copied
-    m_uniqueId(layer.m_uniqueId),
-    m_relativeFixedLayer(0)
+    m_uniqueId(layer.m_uniqueId)
 {
     m_doRotation = layer.m_doRotation;
     m_isFixed = layer.m_isFixed;
-    m_isRelativeTo = layer.m_isRelativeTo;
-    m_relativeFixedLayerID = layer.m_relativeFixedLayerID;
-    m_deltaPosition = layer.m_deltaPosition;
 
     m_angleTransform = layer.m_angleTransform;
     m_translation = layer.m_translation;
@@ -108,14 +100,10 @@ LayerAndroid::LayerAndroid(SkPicture* picture) : SkLayer(),
     m_haveClip(false),
     m_doRotation(false),
     m_isFixed(false),
-    m_isRelativeTo(false),
-    m_relativeFixedLayerID(0),
     m_recordingPicture(picture),
     m_extra(0),
-    m_uniqueId(-1),
-    m_relativeFixedLayer(0)
+    m_uniqueId(-1)
 {
-    m_deltaPosition.set(0, 0);
     m_angleTransform = 0;
     m_translation.set(0, 0);
     m_scale.set(1, 1);
@@ -257,27 +245,6 @@ const LayerAndroid* LayerAndroid::find(int x, int y) const
 
 ///////////////////////////////////////////////////////////////////////////////
 
-void LayerAndroid::setRelativeTo(LayerAndroid* container) {
-    ASSERT(container->m_isFixed);
-    m_relativeFixedLayerID = container->m_uniqueId;
-    m_isRelativeTo = true;
-    m_deltaPosition = getPosition() - container->getPosition();
-}
-
-void LayerAndroid::ensureFixedLayersForDescendants(const LayerAndroid* rootLayer) {
-    if (m_isRelativeTo && !m_relativeFixedLayer) {
-        LayerAndroid* containerLayer = const_cast<LayerAndroid*>(
-            rootLayer->findById(m_relativeFixedLayerID));
-        if (containerLayer)
-            m_relativeFixedLayer = containerLayer;
-    }
-
-    int count = countChildren();
-    for (int i = 0; i < count; i++) {
-        getChild(i)->ensureFixedLayersForDescendants(rootLayer);
-    }
-}
-
 void LayerAndroid::updateFixedLayersPositions(const SkRect& viewport) {
 
     if (m_isFixed) {
@@ -311,13 +278,6 @@ void LayerAndroid::updatePositions() {
     // apply the viewport to us
     SkMatrix matrix;
     if (!m_isFixed) {
-        // If we are defined as being relative to a fixed
-        // layer, we need to update our position...
-        if (m_isRelativeTo && m_relativeFixedLayer) {
-            this->setPosition(m_relativeFixedLayer->getPosition().fX + m_deltaPosition.fX,
-                m_relativeFixedLayer->getPosition().fY + m_deltaPosition.fY);
-        }
-
         // turn our fields into a matrix.
         //
         // TODO: this should happen in the caller, and we should remove these
diff --git a/WebCore/platform/graphics/android/LayerAndroid.h b/WebCore/platform/graphics/android/LayerAndroid.h
index 005ef3d..247e902 100644
--- a/WebCore/platform/graphics/android/LayerAndroid.h
+++ b/WebCore/platform/graphics/android/LayerAndroid.h
@@ -103,14 +103,6 @@ public:
         m_isFixed = true;
     }
 
-    /** Call that method to position the layer relative to another one.
-        We can only be set relative to a fixed layer.
-        The parameter is not refcounted -- we only save its ID as we
-        need to reconnect with the correct layer once we copy the tree
-        to the UI.
-    */
-    void setRelativeTo(LayerAndroid* container);
-
     void setBackgroundColor(SkColor color);
     void setMaskLayer(LayerAndroid*);
     void setMasksToBounds(bool);
@@ -134,16 +126,6 @@ public:
     void dumpLayers(FILE*, int indentLevel) const;
     void dumpToLog() const;
 
-    /** Call this to be sure all fixed descendants correctly have
-        a pointer to their container layer before we try
-        to update the positions. The fixed layer we point to is
-        not refcounted (no need, it's already in the layers' tree).
-
-        This call is recursive, so it should be called on the root of the
-        hierarchy.
-    */
-    void ensureFixedLayersForDescendants(const LayerAndroid* rootLayer);
-
     /** Call this with the current viewport (scrolling, zoom) to update
         the position of the fixed layers.
 
@@ -185,7 +167,6 @@ private:
     bool m_haveClip;
     bool m_doRotation;
     bool m_isFixed;
-    bool m_isRelativeTo;
     bool m_backgroundColorSet;
 
     SkLength m_fixedLeft;
@@ -194,8 +175,6 @@ private:
     SkLength m_fixedBottom;
     int m_fixedWidth;
     int m_fixedHeight;
-    int m_relativeFixedLayerID;
-    SkPoint m_deltaPosition;
 
     SkPoint m_translation;
     SkPoint m_scale;
@@ -209,8 +188,6 @@ private:
     DrawExtra* m_extra;
     int m_uniqueId;
 
-    LayerAndroid* m_relativeFixedLayer;
-
     typedef SkLayer INHERITED;
 };