Simply rules for compositing fixed position elements

Source/core/rendering/compositing/CompositingReasonFinder.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/rendering/compositing/CompositingReasonFinder.h"
 
 #include "core/CSSPropertyNames.h"
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
@@ skipping 133 matching lines @@
 
         if (const RenderLayer* scrollingAncestor = layer->ancestorScrollingLayer()) {
             if (scrollingAncestor->needsCompositedScrolling() && layer->scrollParent())
                 directReasons |= CompositingReasonOverflowScrollingParent;
         }
 
         if (layer->needsCompositedScrolling())
             directReasons |= CompositingReasonOverflowScrollingTouch;
     }
 
-    if (requiresCompositingForPositionFixed(renderer, layer, 0))
+    if (requiresCompositingForPositionFixed(renderer))
         directReasons |= CompositingReasonPositionFixed;
 
     directReasons |= renderer->additionalCompositingReasons(m_compositingTriggers);
 
     ASSERT(!(directReasons & CompositingReasonComboAllStyleDeterminedReasons));
     return directReasons;
 }
 
 bool CompositingReasonFinder::requiresCompositingForAnimation(RenderStyle* style) const
 {
     if (style->subtreeWillChangeContents())
         return style->isRunningAnimationOnCompositor();
 
     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;

[Ian Vollick, 2014/08/07 15:56:15]

The CompositingInputsUpdater could be trivially updated to compute
has-a-scroller-on-the-parent-chain. Maybe we could add this and still avoid
promoting when when nothing's scrolling?

[abarth-chromium, 2014/08/07 17:32:04]

This check doesn't really make sense.  We only composite fixed position elements
that have the viewport as their container.  Why does it matter whether there's
anything scrollable on their RenderLayer parent chain?  We could check whether
the viewport scrolls overflow if you like.  That makes sense.

[Ian Vollick, 2014/08/07 17:44:26]

Sorry, yup, you're right - the situation I was imagining is bogus. I do like the
idea of avoiding promotion when the viewport can't scroll.

-        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;

[Ian Vollick, 2014/08/07 15:56:15]

I'm glad this is going - I think this should have been handled elsewhere.
There's no reason that fixed position elements should be special wrt visibility.

-        return false;
-    }
-
-    // Fixed position elements that are invisible in the current view don't get their own layer.

[Ian Vollick, 2014/08/07 15:56:15]

This also seems like a nice chunk to remove -- I think that viewporting is the
correct, general solution for this special case.

-    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;
 }
 
 }