Record non-stacking-context as main thread scrolling reasons

cc/input/main_thread_scrolling_reason.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 #define CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 
 #include <string>
 
 #include "base/stl_util.h"
@@ skipping 26 matching lines @@
     // These *AndLCDText reasons are due to subpixel text rendering which can
     // only be applied by blending glyphs with the background at a specific
     // screen position; transparency and transforms break this.
     kNonCompositedReasonsFirst = 16,
     kHasOpacityAndLCDText = 1 << 16,
     kHasTransformAndLCDText = 1 << 17,
     kBackgroundNotOpaqueInRectAndLCDText = 1 << 18,
     kHasBorderRadius = 1 << 19,
     kHasClipRelatedProperty = 1 << 20,
     kHasBoxShadowFromNonRootLayer = 1 << 21,
-    kNonCompositedReasonsLast = 21,
+    kIsNotStackingContextAndLCDText = 1 << 22,
+    kHasOverflowClipAndLCDText = 1 << 23,

[bokan, 2017/04/25 18:04:15]

I think this name should be more specific - having an overflow clip isn't enough
to have this reason set.

[yigu, 2017/04/26 15:38:36]

Updated. It also requires that the layout object's background is not opaque,
e.g.  it has alpha channel.

+    kNonCompositedReasonsLast = 23,
 
     // Transient scrolling reasons. These are computed for each scroll begin.
     kNonFastScrollableRegion = 1 << 5,
     kFailedHitTest = 1 << 7,
     kNoScrollingLayer = 1 << 8,
     kNotScrollable = 1 << 9,
     kContinuingMainThreadScroll = 1 << 10,
     kNonInvertibleTransform = 1 << 11,
     kPageBasedScrolling = 1 << 12,
 
     // The maximum number of flags in this struct (excluding itself).
     // New flags should increment this number but it should never be decremented
     // because the values are used in UMA histograms. It should also be noted
     // that it excludes the kNotScrollingOnMain value.
-    kMainThreadScrollingReasonCount = 22,
+    kMainThreadScrollingReasonCount = 24,
   };
 
   static const uint32_t kNonCompositedReasons =
       kHasOpacityAndLCDText | kHasTransformAndLCDText |
       kBackgroundNotOpaqueInRectAndLCDText | kHasBorderRadius |
-      kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer;
+      kHasClipRelatedProperty | kHasBoxShadowFromNonRootLayer |
+      kIsNotStackingContextAndLCDText | kHasOverflowClipAndLCDText;
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
   static bool MainThreadCanSetScrollReasons(uint32_t reasons) {
     uint32_t reasons_set_by_main_thread =
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
         kScrollbarScrolling | kPageOverlay | kHandlingScrollFromMainThread |
         kCustomScrollbarScrolling;
     return (reasons & reasons_set_by_main_thread) == reasons;
@@ skipping 54 matching lines @@
         MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText) {
       tracedValue->AppendString(
           "Background is not opaque in rect and LCD text");
     }
     if (reasons & MainThreadScrollingReason::kHasBorderRadius)
       tracedValue->AppendString("Has border radius");
     if (reasons & MainThreadScrollingReason::kHasClipRelatedProperty)
       tracedValue->AppendString("Has clip related property");
     if (reasons & MainThreadScrollingReason::kHasBoxShadowFromNonRootLayer)
       tracedValue->AppendString("Has box shadow from non-root layer");
+    if (reasons & MainThreadScrollingReason::kIsNotStackingContextAndLCDText)
+      tracedValue->AppendString("Is not stacking context and LCD text");
+    if (reasons & MainThreadScrollingReason::kHasOverflowClipAndLCDText)
+      tracedValue->AppendString("Has overflow clip and LCD text");
 
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)
       tracedValue->AppendString("Non fast scrollable region");
     if (reasons & MainThreadScrollingReason::kFailedHitTest)
       tracedValue->AppendString("Failed hit test");
     if (reasons & MainThreadScrollingReason::kNoScrollingLayer)
       tracedValue->AppendString("No scrolling layer");
     if (reasons & MainThreadScrollingReason::kNotScrollable)
       tracedValue->AppendString("Not scrollable");
     if (reasons & MainThreadScrollingReason::kContinuingMainThreadScroll)
       tracedValue->AppendString("Continuing main thread scroll");
     if (reasons & MainThreadScrollingReason::kNonInvertibleTransform)
       tracedValue->AppendString("Non-invertible transform");
     if (reasons & MainThreadScrollingReason::kPageBasedScrolling)
       tracedValue->AppendString("Page-based scrolling");
     tracedValue->EndArray();
   }
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_