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

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

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 148 matching lines @@
 static bool RequiresCompositingOrSquashing(CompositingReasons reasons) {
 #if DCHECK_IS_ON()
   bool fast_answer = reasons != kCompositingReasonNone;
   bool slow_answer = RequiresCompositing(reasons) || RequiresSquashing(reasons);
   DCHECK_EQ(slow_answer, fast_answer);
 #endif
   return reasons != kCompositingReasonNone;
 }
 
 static CompositingReasons SubtreeReasonsForCompositing(
+    const CompositingReasonFinder& compositing_reason_finder,
     PaintLayer* layer,
     bool has_composited_descendants,
     bool has3d_transformed_descendants) {
   CompositingReasons subtree_reasons = kCompositingReasonNone;
 
   // When a layer has composited descendants, some effects, like 2d transforms,
   // filters, masks etc must be implemented via compositing so that they also
   // apply to those composited descendants.
   if (has_composited_descendants) {
     subtree_reasons |= layer->PotentialCompositingReasonsFromStyle() &
                        kCompositingReasonComboCompositedDescendants;
 
     if (layer->ShouldIsolateCompositedDescendants()) {
       DCHECK(layer->StackingNode()->IsStackingContext());
       subtree_reasons |= kCompositingReasonIsolateCompositedDescendants;
     }
 
     // FIXME: This should move into
     // CompositingReasonFinder::potentialCompositingReasonsFromStyle, but theres
     // a poor interaction with LayoutTextControlSingleLine, which sets this
     // hasOverflowClip directly.
     if (layer->GetLayoutObject().HasClipRelatedProperty())
       subtree_reasons |= kCompositingReasonClipsCompositingDescendants;
 
-    if (layer->GetLayoutObject().Style()->GetPosition() == EPosition::kFixed)
+    // We ignore LCD text here because we are required to composite
+    // scroll-dependant fixed position elements with composited descendants for
+    // correctness - even if we lose LCD.
+    const bool ignore_lcd_text = true;
+    if (layer->GetLayoutObject().Style()->GetPosition() == EPosition::kFixed &&

[chrishtr, 2017/05/03 19:03:31]

The change here will affect fixed-position elements that are not fixed to the
viewport
also, correct? Intended?

[smcgruer, 2017/05/04 19:28:52]

Intended, yes. My understanding was that we shouldn't need to composite in that
case. I'm welcome to being corrected if wrong :)

+        compositing_reason_finder.RequiresCompositingForScrollDependentPosition(
+            layer, ignore_lcd_text)) {
       subtree_reasons |=
           kCompositingReasonPositionFixedWithCompositedDescendants;
+    }
   }
 
   // A layer with preserve-3d or perspective only needs to be composited if
   // there are descendant layers that will be affected by the preserve-3d or
   // perspective.
   if (has3d_transformed_descendants) {
     subtree_reasons |= layer->PotentialCompositingReasonsFromStyle() &
                        kCompositingReasonCombo3DDescendants;
   }
 
@@ skipping 284 matching lines @@
     // into their compositing ancestor's backing, and so are still considered
     // for overlap.
     if (child_recursion_data.compositing_ancestor_ &&
         !child_recursion_data.compositing_ancestor_->IsRootLayer())
       overlap_map.Add(layer, abs_bounds, !has_composited_scrolling_ancestor);
 
     // Now check for reasons to become composited that depend on the state of
     // 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 &&
         RequiresCompositingOrSquashing(subtree_compositing_reasons)) {
       child_recursion_data.compositing_ancestor_ = layer;
       // FIXME: this context push is effectively a no-op but needs to exist for
       // now, because the code is designed to push overlap information to the
       // second-from-top context of the stack.
       overlap_map.BeginNewOverlapTestingContext();
       overlap_map.Add(layer, absolute_descendant_bounding_box,
@@ skipping 37 matching lines @@
     descendant_has3d_transform |=
         any_descendant_has3d_transform || layer->Has3DTransform();
   }
 
   // At this point we have finished collecting all reasons to composite this
   // layer.
   layer->SetCompositingReasons(reasons_to_composite);
 }
 
 }  // namespace blink