Add Feature and about:flag for root scroller

This will be used to allow a kill-switch for the feature post-launch as
well as help debugging.

Bug: 903263
Change-Id: Icaf0b9ea4f28f9abddfa67f88fc6759aca812ec3
Reviewed-on: https://chromium-review.googlesource.com/c/1349476Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611404}

chrome/browser/about_flags.cc

@@ -4461,6 +4461,11 @@ const FeatureEntry kFeatureEntries[] = {
     {"google-password-manager", flag_descriptions::kGooglePasswordManagerName,
      flag_descriptions::kGooglePasswordManagerDescription, kOsAll,
      FEATURE_VALUE_TYPE(password_manager::features::kGooglePasswordManager)},
+
+    {"enable-implicit-root-scroller",
+     flag_descriptions::kEnableImplicitRootScrollerName,
+     flag_descriptions::kEnableImplicitRootScrollerDescription, kOsAll,
+     FEATURE_VALUE_TYPE(blink::features::kImplicitRootScroller)},
 };
 
 class FlagsStateSingleton {

chrome/browser/flag-metadata.json

@@ -1303,6 +1303,11 @@
     // "owners": [ "your-team" ],
     "expiry_milestone": 76
   },
+  {
+    "name": "enable-implicit-root-scroller",
+    "owners": [ "bokan" ],
+    "expiry_milestone": 75
+  },
   {
     "name": "enable-improved-language-settings",
     // "owners": [ "your-team" ],

chrome/browser/flag_descriptions.cc

@@ -580,6 +580,12 @@ const char kEnableHeavyPageCappingDescription[] =
     "will reduce the triggering threshold to 1MB. Data saver must be enabled "
     "in order to see the InfoBar.";
 
+const char kEnableImplicitRootScrollerName[] = "Implicit Root Scroller";
+const char kEnableImplicitRootScrollerDescription[] =
+    "Enables implicitly choosing which scroller on a page is the 'root "
+    "scroller'. i.e. The one that gets special features like URL bar movement, "
+    "overscroll glow, rotation anchoring, etc.";
+
 const char kEnablePreviewsAndroidOmniboxUIName[] =
     "Previews Android Omnibox UI";
 const char kEnablePreviewsAndroidOmniboxUIDescription[] =

chrome/browser/flag_descriptions.h

@@ -374,6 +374,9 @@ extern const char kEnableHDRDescription[];
 extern const char kEnableHeavyPageCappingName[];
 extern const char kEnableHeavyPageCappingDescription[];
 
+extern const char kEnableImplicitRootScrollerName[];
+extern const char kEnableImplicitRootScrollerDescription[];
+
 extern const char kEnablePreviewsAndroidOmniboxUIName[];
 extern const char kEnablePreviewsAndroidOmniboxUIDescription[];
 

content/child/runtime_features.cc

@@ -501,6 +501,9 @@ void SetRuntimeFeaturesDefaultsAndUpdateFromArgs(
   WebRuntimeFeatures::EnablePortals(
       base::FeatureList::IsEnabled(blink::features::kPortals));
 
+  WebRuntimeFeatures::EnableImplicitRootScroller(
+      base::FeatureList::IsEnabled(blink::features::kImplicitRootScroller));
+
   if (!base::FeatureList::IsEnabled(features::kBackgroundFetch))
     WebRuntimeFeatures::EnableBackgroundFetch(false);
 

third_party/blink/common/features.cc

@@ -43,6 +43,10 @@ const base::Feature kOnionSoupDOMStorage{"OnionSoupDOMStorage",
 // Enable Portals. https://crbug.com/865123.
 const base::Feature kPortals{"Portals", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Enable Implicit Root Scroller. https://crbug.com/903260.
+const base::Feature kImplicitRootScroller{"ImplicitRootScroller",
+                                          base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Used to control the collection of anchor element metrics (crbug.com/856683).
 // If kRecordAnchorMetricsClicked is enabled, then metrics of anchor elements
 // clicked by the user will be extracted and recorded.

third_party/blink/public/common/features.h

@@ -21,6 +21,7 @@ BLINK_COMMON_EXPORT extern const base::Feature kLayoutNG;
 BLINK_COMMON_EXPORT extern const base::Feature kMojoBlobURLs;
 BLINK_COMMON_EXPORT extern const base::Feature kOnionSoupDOMStorage;
 BLINK_COMMON_EXPORT extern const base::Feature kPortals;
+BLINK_COMMON_EXPORT extern const base::Feature kImplicitRootScroller;
 BLINK_COMMON_EXPORT extern const base::Feature kRecordAnchorMetricsClicked;
 BLINK_COMMON_EXPORT extern const base::Feature kRecordAnchorMetricsVisible;
 BLINK_COMMON_EXPORT extern const base::Feature

third_party/blink/public/platform/web_runtime_features.h

@@ -108,6 +108,7 @@ class WebRuntimeFeatures {
   BLINK_PLATFORM_EXPORT static void EnableGenericSensor(bool);
   BLINK_PLATFORM_EXPORT static void EnableGenericSensorExtraClasses(bool);
   BLINK_PLATFORM_EXPORT static void EnableHeapCompaction(bool);
+  BLINK_PLATFORM_EXPORT static void EnableImplicitRootScroller(bool);
   BLINK_PLATFORM_EXPORT static void EnableInputMultipleFieldsUI(bool);
   BLINK_PLATFORM_EXPORT static void EnableJankTracking(bool);
   BLINK_PLATFORM_EXPORT static void EnableLayeredAPI(bool);

third_party/blink/renderer/platform/exported/web_runtime_features.cc

@@ -192,6 +192,10 @@ void WebRuntimeFeatures::EnableHeapCompaction(bool enable) {
   RuntimeEnabledFeatures::SetHeapCompactionEnabled(enable);
 }
 
+void WebRuntimeFeatures::EnableImplicitRootScroller(bool enable) {
+  RuntimeEnabledFeatures::SetImplicitRootScrollerEnabled(enable);
+}
+
 void WebRuntimeFeatures::EnableInputMultipleFieldsUI(bool enable) {
   RuntimeEnabledFeatures::SetInputMultipleFieldsUIEnabled(enable);
 }

tools/metrics/histograms/enums.xml

@@ -30638,6 +30638,7 @@ from previous Chrome versions.
   <int value="537857256" label="PipRoundedCorners:enabled"/>
   <int value="538468149" label="OfflinePagesCT:enabled"/>
   <int value="538600423" label="OmniboxDocumentProvider:enabled"/>
+  <int value="546043947" label="ImplicitRootScroller:enabled"/>
   <int value="546520086" label="enable-data-reduction-proxy-savings-promo"/>
   <int value="546710806" label="disable-easy-signin"/>
   <int value="549483647" label="EnableUnifiedMultiDeviceSettings:disabled"/>
@@ -31419,6 +31420,7 @@ from previous Chrome versions.
   <int value="1913298816" label="OverlayScrollbar:enabled"/>
   <int value="1913926782" label="ChromeModernAlternateCardLayout:disabled"/>
   <int value="1915178511" label="disable-blink-features"/>
+  <int value="1919917329" label="ImplicitRootScroller:disabled"/>
   <int value="1924192543" label="ProactiveTabFreezeAndDiscard:enabled"/>
   <int value="1925627218" label="FullscreenToolbarReveal:disabled"/>
   <int value="1926524951" label="SystemWebApps:disabled"/>