Simply rules for compositing fixed position elements

Source/core/rendering/compositing/CompositingReasonFinder.cpp

Index: Source/core/rendering/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/rendering/compositing/CompositingReasonFinder.cpp b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
index 7c3043b65c0058e565458ad535309ba15829a5d7..bae98d2cffcb571e10d78a1edd41acb6d01bb3bf 100644
--- a/Source/core/rendering/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
@@ -151,7 +151,7 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
             directReasons |= CompositingReasonOverflowScrollingTouch;
     }
 
-    if (requiresCompositingForPositionFixed(renderer, layer, 0))
+    if (requiresCompositingForPositionFixed(renderer))
         directReasons |= CompositingReasonPositionFixed;
 
     directReasons |= renderer->additionalCompositingReasons(m_compositingTriggers);
@@ -168,84 +168,15 @@ bool CompositingReasonFinder::requiresCompositingForAnimation(RenderStyle* style
     return style->shouldCompositeForCurrentAnimations();
 }
 
-bool CompositingReasonFinder::requiresCompositingForPositionFixed(RenderObject* renderer, const RenderLayer* layer, RenderLayer::ViewportConstrainedNotCompositedReason* viewportConstrainedNotCompositedReason) const
+bool CompositingReasonFinder::requiresCompositingForPositionFixed(RenderObject* renderer) const
 {
     if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger))
         return false;
-
-    if (renderer->style()->position() != FixedPosition)
-        return false;
-
-    RenderObject* container = renderer->container();
-    // If the renderer is not hooked up yet then we have to wait until it is.
-    if (!container) {
-        ASSERT(m_renderView.document().lifecycle().state() < DocumentLifecycle::InCompositingUpdate);
-        // FIXME: Remove this and ASSERT(container) once we get rid of the incremental
-        // allocateOrClearCompositedLayerMapping compositing update. This happens when
-        // adding the renderer to the tree because we setStyle before addChild in
-        // createRendererForElementIfNeeded.
-        return false;
-    }
-
     // Don't promote fixed position elements that are descendants of a non-view container, e.g. transformed elements.
     // They will stay fixed wrt the container rather than the enclosing frame.
-    if (container != &m_renderView) {
-        if (viewportConstrainedNotCompositedReason)
-            *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForNonViewContainer;
-        return false;
-    }
-
-    // If the fixed-position element does not have any scrollable ancestor between it and
-    // its container, then we do not need to spend compositor resources for it. Start by
-    // assuming we can opt-out (i.e. no scrollable ancestor), and refine the answer below.
-    bool hasScrollableAncestor = false;
-
-    // The FrameView has the scrollbars associated with the top level viewport, so we have to
-    // check the FrameView in addition to the hierarchy of ancestors.
-    FrameView* frameView = m_renderView.frameView();
-    if (frameView && frameView->isScrollable())
-        hasScrollableAncestor = true;
-
-    RenderLayer* ancestor = layer->parent();
-    while (ancestor && !hasScrollableAncestor) {
-        if (ancestor->scrollsOverflow())
-            hasScrollableAncestor = true;
-        if (ancestor->renderer() == &m_renderView)
-            break;
-        ancestor = ancestor->parent();
-    }
-
-    if (!hasScrollableAncestor) {
-        if (viewportConstrainedNotCompositedReason)
-            *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForUnscrollableAncestors;
-        return false;
-    }
-
-    // Subsequent tests depend on layout. If we can't tell now, just keep things the way they are until layout is done.
-    // FIXME: Get rid of this codepath once we get rid of the incremental compositing update in RenderLayer::styleChanged.
-    if (m_renderView.document().lifecycle().state() < DocumentLifecycle::LayoutClean)
-        return layer->hasCompositedLayerMapping();
-
-    bool paintsContent = layer->isVisuallyNonEmpty() || layer->hasVisibleDescendant();
-    if (!paintsContent) {
-        if (viewportConstrainedNotCompositedReason)
-            *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForNoVisibleContent;
-        return false;
-    }
-
-    // Fixed position elements that are invisible in the current view don't get their own layer.
-    if (FrameView* frameView = m_renderView.frameView()) {
-        ASSERT(m_renderView.document().lifecycle().state() == DocumentLifecycle::InCompositingUpdate);
-        LayoutRect viewBounds = frameView->viewportConstrainedVisibleContentRect();
-        LayoutRect layerBounds = layer->boundingBoxForCompositing(layer->compositor()->rootRenderLayer(), RenderLayer::ApplyBoundsChickenEggHacks);
-        if (!viewBounds.intersects(enclosingIntRect(layerBounds))) {
-            if (viewportConstrainedNotCompositedReason)
-                *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForBoundsOutOfView;
-            return false;
-        }
-    }
-
-    return true;
+    return renderer->style()->position() == FixedPosition
+        && renderer->container() == &m_renderView
+        && m_renderView.frameView()->isScrollable();
 }
 
 }