Currently we only composite fixed position elements if we are on High DPI devices. This fix allows …

Source/core/layout/compositing/CompositingReasonFinder.cpp

Index: Source/core/layout/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/layout/compositing/CompositingReasonFinder.cpp b/Source/core/layout/compositing/CompositingReasonFinder.cpp
index 872073d7418ac37fd9f1e91434c25268fd0d3432..1e6df4c0418dd90ffc5e0d341aa30f249171056d 100644
--- a/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -176,11 +176,17 @@ bool CompositingReasonFinder::requiresCompositingForAnimation(const ComputedStyl
 
 bool CompositingReasonFinder::requiresCompositingForPositionFixed(const DeprecatedPaintLayer* layer) const
 {
-    if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger))
-        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.
-    return layer->scrollsWithViewport() && m_layoutView.frameView()->isScrollable();
+    if (!layer->scrollsWithViewport() || !m_layoutView.frameView()->isScrollable())
+        return false;
+
+    if (m_compositingTriggers & ViewportConstrainedPositionedTrigger)
+        return true;
+
+    // If the layer is opaque we can still promote it since we will not lose LCD text
+    LayoutRect bounds = layer->layoutBox()->visualOverflowRect();

[trchen, 2015/08/27 23:10:58]

I looked at the semantics of visualOverflowRect(), it seems that it only
includes the visual overflow from non-stacking-context descendants. (See
LayoutBox::addOverflowFromChild)

I suspect the logic is going to work for this example:
<div style="position:fixed; z-index:0; width:1px; height:1px;
background:white;">
  <div style="position:relative; z-index:0;">
    The text is probably not counted towards visualOverflowRect of the fixed-pos
parent.
  </div>
</div>

This is an awkward situation because in CompositedDeprecatedPaintLayerMapping we
use backgroundIsKnownToBeOpaqueInRect(compositedBounds()) to decide whether a
layer is opaque, but the compositedBounds() (i.e. The union of visual overflow
rect of non-composited descendants.) can't be calculated at this point because
compositing decision hasn't been made yet.

I think the best thing you can do is compute the union of layer bounding box of
all descendant DPLs, ignoring compositing status. You can refer to the code in
DeprecatedPaintLayer::physicalBoundingBoxIncludingReflectionAndStackingChildren().
Perhaps add a enum option to it to ignore compositing status?

[davidfox, 2015/08/30 22:00:04]

Wouldn't it be possible to call DeprecatedPaintLayer::boundingBoxForCompositing
directly?

+    return layer->backgroundIsKnownToBeOpaqueInRect(bounds);
 }
 
 }