[CompositeAfterPaint] Composited scrolling of view based on html/body's hint

CSS scrolling properties on html/body apply to the view. For example,
overflow:hidden on html or body actually means the view is not
scrollable. Based on this, using compositing hint on html/body for the
view composited scrolling is reasonable. This seems a better way to hint
composited scrolling of an iframe's content than hinting on the iframe
because this can be controlled by the content document.

Change-Id: I78c3683d1c6f3b610161f5ebfb5cd7d2f6129b11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125907Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754565}

third_party/blink/renderer/core/paint/compositing/compositing_reason_finder.cc

@@ -10,6 +10,7 @@
 #include "third_party/blink/renderer/core/frame/local_frame_view.h"
 #include "third_party/blink/renderer/core/frame/settings.h"
 #include "third_party/blink/renderer/core/fullscreen/fullscreen.h"
+#include "third_party/blink/renderer/core/html/html_body_element.h"
 #include "third_party/blink/renderer/core/layout/layout_embedded_content.h"
 #include "third_party/blink/renderer/core/layout/layout_video.h"
 #include "third_party/blink/renderer/core/layout/layout_view.h"
@@ -103,6 +104,26 @@ CompositingReasonFinder::PotentialCompositingReasonsFromStyle(
   return reasons;
 }
 
+static bool ShouldPreferCompositingForLayoutView(
+    const LayoutView& layout_view) {
+  auto has_direct_compositing_reasons = [](const LayoutObject* object) -> bool {
+    return object && CompositingReasonFinder::DirectReasonsForPaintProperties(
+                         *object) != CompositingReason::kNone;
+  };
+  if (has_direct_compositing_reasons(
+          layout_view.GetFrame()->OwnerLayoutObject()))
+    return true;
+  if (auto* document_element = layout_view.GetDocument().documentElement()) {
+    if (has_direct_compositing_reasons(document_element->GetLayoutObject()))
+      return true;
+  }
+  if (auto* body = layout_view.GetDocument().FirstBodyElement()) {
+    if (has_direct_compositing_reasons(body->GetLayoutObject()))
+      return true;
+  }
+  return false;
+}
+
 CompositingReasons CompositingReasonFinder::DirectReasonsForPaintProperties(
     const LayoutObject& object) {
   // TODO(wangxianzhu): Don't depend on PaintLayer for CompositeAfterPaint.
@@ -143,15 +164,9 @@ CompositingReasons CompositingReasonFinder::DirectReasonsForPaintProperties(
           // a direct compositing reason, it's very likely that the object will
           // be composited, and it also indicates preference of compositing,
           // so we prefer composited scrolling here.
-          style.BackfaceVisibility() == EBackfaceVisibility::kHidden;
-
-      if (!force_prefer_compositing_to_lcd_text && object.IsLayoutView()) {
-        if (auto* owner_object = object.GetFrame()->OwnerLayoutObject()) {
-          force_prefer_compositing_to_lcd_text =
-              DirectReasonsForPaintProperties(*owner_object) !=
-              CompositingReason::kNone;
-        }
-      }
+          style.BackfaceVisibility() == EBackfaceVisibility::kHidden ||
+          (object.IsLayoutView() &&
+           ShouldPreferCompositingForLayoutView(To<LayoutView>(object)));
 
       if (scrollable_area->ComputeNeedsCompositedScrolling(
               force_prefer_compositing_to_lcd_text)) {

third_party/blink/web_tests/FlagExpectations/composite-after-paint

@@ -80,8 +80,6 @@ compositing/overflow/do-not-repaint-if-scrolling-composited-layers.html [ Failur
 
 # We paint the iframe's content background in the scrolling layer, causing invalidation on scroll.
 paint/invalidation/scroll/iframe-scroll-repaint.html [ Failure ]
-# will-transform on descendant doesn't trigger compositing of iframe.
-paint/invalidation/scroll/composited-iframe-scroll-repaint.html [ Failure ]
 
 # Extra layers for non-fast scrolling areas.
 compositing/overflow/textarea-scroll-touch.html [ Failure ]

third_party/blink/web_tests/flag-specific/composite-after-paint/paint/invalidation/scroll/composited-iframe-scroll-repaint-expected.txt

+{
+  "layers": [
+    {
+      "name": "Scrolling background of LayoutView #document",
+      "bounds": [800, 600],
+      "contentsOpaque": true,
+      "backgroundColor": "#FFFFFF"
+    },
+    {
+      "name": "Scrolling background of LayoutView #document",
+      "bounds": [300, 516],
+      "contentsOpaque": true,
+      "backgroundColor": "#EEEEEE",
+      "transform": 2
+    },
+    {
+      "name": "LayoutNGBlockFlow BODY",
+      "bounds": [284, 500],
+      "drawsContent": false,
+      "transform": 3
+    }
+  ],
+  "transforms": [
+    {
+      "id": 1,
+      "transform": [
+        [1, 0, 0, 0],
+        [0, 1, 0, 0],
+        [0, 0, 1, 0],
+        [10, 10, 0, 1]
+      ]
+    },
+    {
+      "id": 2,
+      "parent": 1,
+      "transform": [
+        [1, 0, 0, 0],
+        [0, 1, 0, 0],
+        [0, 0, 1, 0],
+        [0, -20, 0, 1]
+      ]
+    },
+    {
+      "id": 3,
+      "parent": 2,
+      "transform": [
+        [1, 0, 0, 0],
+        [0, 1, 0, 0],
+        [0, 0, 1, 0],
+        [8, 8, 0, 1]
+      ]
+    }
+  ]
+}
+