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..a9eea942165e36647f31a79e93a217999b22aac1 100644
--- a/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -176,11 +176,18 @@ 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)) {

[trchen, 2015/08/14 21:02:17]

Please swap the branches. Avoid negative predicates and short branch goes first.
:)

+        // If the layer is opaque we can still promote it since we will not lose LCD text
+        LayoutRect bounds = layer->layoutBox()->contentBoxRect();

[trchen, 2015/08/14 21:02:17]

This doesn't look right because texts can overflow. Better simply look at the
entire visual overflow.

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