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

BUG=715699
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2839263002
Cr-Commit-Position: refs/heads/master@{#469584}

third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-features=LayoutNG

@@ -548,7 +548,6 @@ crbug.com/591099 canvas/philip/tests/2d.shadow.stroke.cap.1.html [ Crash Pass ]
 crbug.com/591099 canvas/philip/tests/size.attributes.parse.em.html [ Crash Pass ]
 crbug.com/591099 canvas/philip/tests/toDataURL.jpeg.primarycolours.html [ Crash Pass ]
 crbug.com/591099 compositing/3d-corners.html [ Failure ]
-crbug.com/591099 compositing/always-composite-fixed-position-when-descendants-composite.html [ Failure ]
 crbug.com/591099 compositing/animation/busy-indicator.html [ Failure ]
 crbug.com/591099 compositing/animation/hidden-composited.html [ Failure ]
 crbug.com/591099 compositing/backface-visibility/backface-visibility-image.html [ Failure ]
@@ -563,6 +562,7 @@ crbug.com/591099 compositing/change-preferCompositingToLCDText-setting.html [ Fa
 crbug.com/591099 compositing/checkerboard.html [ Failure ]
 crbug.com/591099 compositing/child-transform-layer-requires-box.html [ Failure ]
 crbug.com/591099 compositing/columns/geometry-map-paginated-assert.html [ Failure ]
+crbug.com/591099 compositing/composite-scrollable-fixed-position-when-descendants-composite.html [ Failure ]
 crbug.com/591099 compositing/composited-negative-zindex-child.html [ Failure ]
 crbug.com/591099 compositing/compositing-visible-descendant.html [ Failure ]
 crbug.com/591099 compositing/contents-opaque/body-background-painted.html [ Failure ]
@@ -20924,7 +20924,6 @@ crbug.com/591099 virtual/android/fullscreen/video-fail-to-enter-full-screen.html
 crbug.com/591099 virtual/android/fullscreen/video-fixed-at-top-left.html [ Failure ]
 crbug.com/591099 virtual/android/media/mediadocument/media-document-with-download-button.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/3d-corners.html [ Failure ]
-crbug.com/591099 virtual/disable-spinvalidation/compositing/always-composite-fixed-position-when-descendants-composite.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/animation/busy-indicator.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/animation/hidden-composited.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/backface-visibility/backface-visibility-image.html [ Failure ]
@@ -20938,6 +20937,7 @@ crbug.com/591099 virtual/disable-spinvalidation/compositing/checkerboard.html [
 crbug.com/591099 virtual/disable-spinvalidation/compositing/child-transform-layer-requires-box.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/child-transform-layer-rounding.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/columns/geometry-map-paginated-assert.html [ Failure ]
+crbug.com/591099 virtual/disable-spinvalidation/compositing/composite-scrollable-fixed-position-when-descendants-composite.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/composited-negative-zindex-child.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/compositing-visible-descendant.html [ Failure ]
 crbug.com/591099 virtual/disable-spinvalidation/compositing/contents-opaque/body-background-painted.html [ Failure ]

third_party/WebKit/LayoutTests/FlagExpectations/enable-slimming-paint-v2

@@ -110,7 +110,6 @@ Bug(none) virtual/without-smil/ [ Skip ]
 
 Bug(none) compositing/3d-cube.html [ Failure ]
 Bug(none) compositing/absolute-inside-out-of-view-fixed.html [ Failure ]
-Bug(none) compositing/always-composite-fixed-position-when-descendants-composite.html [ Failure ]
 Bug(none) compositing/animation/hidden-composited.html [ Failure ]
 Bug(none) compositing/backface-visibility/backface-visibility-image.html [ Failure ]
 Bug(none) compositing/backface-visibility/backface-visibility-webgl.html [ Failure ]
@@ -120,6 +119,7 @@ Bug(none) compositing/change-preferCompositingToLCDText-setting.html [ Failure ]
 Bug(none) compositing/clip-child-by-non-stacking-ancestor.html [ Failure ]
 Bug(none) compositing/color-matching/image-color-matching.html [ Failure ]
 Bug(none) compositing/columns/composited-in-paginated.html [ Failure ]
+Bug(none) compositing/composite-scrollable-fixed-position-when-descendants-composite.html [ Failure ]
 Bug(none) compositing/contents-opaque/background-clip.html [ Failure ]
 Bug(none) compositing/contents-opaque/background-color.html [ Failure ]
 Bug(none) compositing/contents-opaque/body-background-painted.html [ Failure ]

third_party/WebKit/LayoutTests/FlagExpectations/root-layer-scrolls

@@ -2,11 +2,11 @@ Bug(none) accessibility/scroll-to-global-point-main-window.html [ Failure ]
 Bug(none) accessibility/scroll-to-global-point-nested.html [ Failure ]
 Bug(none) accessibility/scroll-to-make-visible-nested-2.html [ Failure ]
 Bug(none) compositing/absolute-inside-out-of-view-fixed.html [ Failure ]
-Bug(none) compositing/always-composite-fixed-position-when-descendants-composite.html [ Failure ]
 Bug(none) compositing/background-color/background-color-outside-document.html [ Failure ]
 Bug(none) compositing/checkerboard.html [ Failure ]
 Bug(none) compositing/clip-child-by-non-stacking-ancestor.html [ Failure ]
 Bug(none) compositing/columns/composited-in-paginated.html [ Failure ]
+Bug(none) compositing/composite-scrollable-fixed-position-when-descendants-composite.html [ Failure ]
 Bug(none) compositing/contents-opaque/background-clip.html [ Failure ]
 Bug(none) compositing/contents-opaque/background-color.html [ Failure ]
 Bug(none) compositing/contents-opaque/body-background-painted.html [ Failure ]
@@ -1249,7 +1249,7 @@ Bug(none) transforms/2d/transform-fixed-container.html [ Failure ]
 Bug(none) transforms/selection-bounds-in-transformed-view.html [ Failure ]
 Bug(none) transforms/transformed-document-element.html [ Failure ]
 Bug(none) virtual/disable-spinvalidation/compositing/absolute-inside-out-of-view-fixed.html [ Failure ]
-Bug(none) virtual/disable-spinvalidation/compositing/always-composite-fixed-position-when-descendants-composite.html [ Failure ]
+Bug(none) virtual/disable-spinvalidation/compositing/composite-scrollable-fixed-position-when-descendants-composite.html [ Failure ]
 Bug(none) virtual/disable-spinvalidation/compositing/background-color/background-color-outside-document.html [ Failure ]
 Bug(none) virtual/disable-spinvalidation/compositing/checkerboard.html [ Failure ]
 Bug(none) virtual/disable-spinvalidation/compositing/clip-child-by-non-stacking-ancestor.html [ Failure ]

third_party/WebKit/LayoutTests/compositing/always-composite-fixed-position-when-descendants-composite.html → third_party/WebKit/LayoutTests/compositing/composite-scrollable-fixed-position-when-descendants-composite.html

 <!doctype HTML>
 Any errors will show below this line.
-<!-- Having a composited child should always be a position:fixed compositing trigger -->
+<!-- Having a composited child should be a position:fixed compositing trigger when scrollable -->
 <div style="position: fixed; width: 100px; height: 100px; background: lightgray">
     <div style="will-change: transform; margin: 50px; width: 50px; height: 50px; background: lightblue">
     </div>
 </div>
+<div style="height: 2000px;"></div>
 <script src="../resources/testharness.js"></script>
 <script>
 if (window.testRunner)

third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.h

@@ -45,16 +45,16 @@ class CORE_EXPORT CompositingReasonFinder {
   static bool RequiresCompositingForTransformAnimation(const ComputedStyle&);
   static bool RequiresCompositingForTransform(const LayoutObject&);
 
+  bool RequiresCompositingForScrollDependentPosition(
+      const PaintLayer*,
+      bool ignore_lcd_text) const;
+
  private:
   bool IsMainFrame() const;
 
   CompositingReasons NonStyleDeterminedDirectReasons(
       const PaintLayer*,
       bool ignore_lcd_text) const;
-  bool RequiresCompositingForScrollDependentPosition(
-      const PaintLayer*,
-      bool ignore_lcd_text) const;
-
   LayoutView& layout_view_;
   CompositingTriggerFlags compositing_triggers_;
 };

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) {
@@ -190,10 +191,17 @@ static CompositingReasons SubtreeReasonsForCompositing(
     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 &&
+        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
@@ -497,7 +505,8 @@ 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 &&