Rename kAnimatingScrollOnMainThread to kHandlingScrollFromMainThread

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/trace_event/trace_event_argument.h"
 
 namespace cc {
 
 // Ensure this stays in sync with MainThreadScrollingReason in histograms.xml.
 // When adding a new MainThreadScrollingReason, make sure the corresponding
 // [MainThread/Compositor]CanSetScrollReasons function is also updated.
 struct MainThreadScrollingReason {
   // Non-transient scrolling reasons.
   enum : uint32_t { kNotScrollingOnMain = 0 };
   enum : uint32_t { kHasBackgroundAttachmentFixedObjects = 1 << 0 };
   enum : uint32_t { kHasNonLayerViewportConstrainedObjects = 1 << 1 };
   enum : uint32_t { kThreadedScrollingDisabled = 1 << 2 };
   enum : uint32_t { kScrollbarScrolling = 1 << 3 };
   enum : uint32_t { kPageOverlay = 1 << 4 };
-  enum : uint32_t { kAnimatingScrollOnMainThread = 1 << 13 };
+  // A scroll handled by the main thread can result in an animation running on
+  // the main thread or on the compositor thread.

[skobes, 2016/09/01 04:00:18]

This comment is a great explanation for why you are renaming the bit, but reads
pretty confusingly as documentation of "what does this bit actually mean". :)  
I'd suggest adding a little more context while you're in here about how and why
we set this.

[ymalik, 2016/09/01 14:24:29]

You're absolutely right. Fixed.

+  enum : uint32_t { kHandlingScrollFromMainThread = 1 << 13 };
   enum : uint32_t { kHasStickyPositionObjects = 1 << 14 };
   enum : uint32_t { kCustomScrollbarScrolling = 1 << 15 };
 
   // Transient scrolling reasons. These are computed for each scroll begin.
   enum : uint32_t { kNonFastScrollableRegion = 1 << 5 };
   enum : uint32_t { kEventHandlers = 1 << 6 };
   enum : uint32_t { kFailedHitTest = 1 << 7 };
   enum : uint32_t { kNoScrollingLayer = 1 << 8 };
   enum : uint32_t { kNotScrollable = 1 << 9 };
   enum : uint32_t { kContinuingMainThreadScroll = 1 << 10 };
   enum : uint32_t { kNonInvertibleTransform = 1 << 11 };
   enum : uint32_t { kPageBasedScrolling = 1 << 12 };
 
   // The number of flags in this struct (excluding itself).
   enum : uint32_t { kMainThreadScrollingReasonCount = 17 };
 
   // 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 | kAnimatingScrollOnMainThread |
+        kScrollbarScrolling | kPageOverlay | kHandlingScrollFromMainThread |
         kHasStickyPositionObjects | kCustomScrollbarScrolling;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }
 
   // Returns true if the given MainThreadScrollingReason can be set by the
   // compositor.
   static bool CompositorCanSetScrollReasons(uint32_t reasons) {
     uint32_t reasons_set_by_compositor =
         kNonFastScrollableRegion | kEventHandlers | kFailedHitTest |
         kNoScrollingLayer | kNotScrollable | kContinuingMainThreadScroll |
@@ skipping 21 matching lines @@
       tracedValue->AppendString("Has background-attachment:fixed");
     if (reasons &
         MainThreadScrollingReason::kHasNonLayerViewportConstrainedObjects)
       tracedValue->AppendString("Has non-layer viewport-constrained objects");
     if (reasons & MainThreadScrollingReason::kThreadedScrollingDisabled)
       tracedValue->AppendString("Threaded scrolling is disabled");
     if (reasons & MainThreadScrollingReason::kScrollbarScrolling)
       tracedValue->AppendString("Scrollbar scrolling");
     if (reasons & MainThreadScrollingReason::kPageOverlay)
       tracedValue->AppendString("Page overlay");
-    if (reasons & MainThreadScrollingReason::kAnimatingScrollOnMainThread)
-      tracedValue->AppendString("Animating scroll on main thread");
+    if (reasons & MainThreadScrollingReason::kHandlingScrollFromMainThread)
+      tracedValue->AppendString("Handling scroll from main thread");
     if (reasons & MainThreadScrollingReason::kHasStickyPositionObjects)
       tracedValue->AppendString("Has sticky position objects");
     if (reasons & MainThreadScrollingReason::kCustomScrollbarScrolling)
       tracedValue->AppendString("Custom scrollbar scrolling");
 
     // Transient scrolling reasons.
     if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)
       tracedValue->AppendString("Non fast scrollable region");
     if (reasons & MainThreadScrollingReason::kEventHandlers)
       tracedValue->AppendString("Event handlers");
     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_