Don't promote position: fixed elements with composited descendants if they don't scroll

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
index 617d570621a6d956d6561ac1812f287b02760740..5d30dc0d7927cbb5c1d89571c0817d02e6bc6dd5 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
@@ -166,6 +166,7 @@ static bool RequiresCompositingOrSquashing(CompositingReasons reasons) {
 }
 
 static CompositingReasons SubtreeReasonsForCompositing(
+    const CompositingReasonFinder& compositing_reason_finder,
     PaintLayer* layer,
     bool has_composited_descendants,
     bool has3d_transformed_descendants) {
@@ -190,9 +191,13 @@ static CompositingReasons SubtreeReasonsForCompositing(
     if (layer->GetLayoutObject().HasClipRelatedProperty())
       subtree_reasons |= kCompositingReasonClipsCompositingDescendants;
 
-    if (layer->GetLayoutObject().Style()->GetPosition() == EPosition::kFixed)
+    const bool ignore_lcd_text = true;

[flackr, 2017/04/27 14:48:47]

nit: Couldn't hurt to add a comment that we ignore LCD text because compositing
is necessary when we have composited descendants for correctness for fixed (and
eventually sticky) position.

[smcgruer, 2017/04/27 14:53:30]

Done.

+    if (layer->GetLayoutObject().Style()->GetPosition() == EPosition::kFixed &&

[flackr, 2017/04/27 14:48:46]

I assume the check for EPosition::kFixed goes away once your other patch lands
right?

[smcgruer, 2017/04/27 14:53:30]

Yep, the other patch will be reduced to removing this check and renaming the
reason constant.

+        compositing_reason_finder.RequiresCompositingForScrollDependentPosition(
+            layer, ignore_lcd_text)) {
       subtree_reasons |=
           kCompositingReasonPositionFixedWithCompositedDescendants;
+    }
   }
 
   // A layer with preserve-3d or perspective only needs to be composited if
@@ -497,7 +502,8 @@ void CompositingRequirementsUpdater::UpdateRecursive(
     // descendant layers.
     CompositingReasons subtree_compositing_reasons =
         SubtreeReasonsForCompositing(
-            layer, child_recursion_data.subtree_is_compositing_,
+            compositing_reason_finder_, layer,
+            child_recursion_data.subtree_is_compositing_,
             any_descendant_has3d_transform);
     reasons_to_composite |= subtree_compositing_reasons;
     if (!will_be_composited_or_squashed && can_be_composited &&