Add ForceLoadAtTop DocumentPolicy

Add a document policy that lets pages opt out of scrolling that
automatically happens on page load. This provides an opt-out to the
Scroll To Text feature (see bug and [1]), but also applies to fragment
scrolls and scroll restoration.

With the DocumentPolicy feature[2] enabled, pages can opt out of these
types of scrolling by adding the HTTP header:
Document-Policy: force-load-at-top

[1] WICG/scroll-to-text-fragment#80
[2] https://github.com/w3c/webappsec-feature-policy/blob/master/document-policy-explainer.md

Bug: 1047900
Change-Id: Iebba7fd24fa2e362bfa4a8f77903763b5d95b787
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2073138
Commit-Queue: Nick Burris <nburris@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748465}

third_party/blink/common/feature_policy/document_policy.cc

@@ -156,6 +156,7 @@ bool DocumentPolicy::IsFeatureSupported(
   switch (feature) {
     case mojom::DocumentPolicyFeature::kFontDisplay:
     case mojom::DocumentPolicyFeature::kUnoptimizedLosslessImages:
+    case mojom::DocumentPolicyFeature::kForceLoadAtTop:
       return true;
     default:
       return false;

third_party/blink/public/mojom/feature_policy/document_policy_feature.mojom

@@ -11,6 +11,8 @@ enum DocumentPolicyFeature {
   kFontDisplay = 1,
   // When disallowed, these policies require images to have a reasonable byte-to-pixel ratio.
   kUnoptimizedLosslessImages = 2,
+  // Controls whether the browser should allow navigations that cause the page to scroll.
+  kForceLoadAtTop = 3,
   // Don't change assigned numbers of any item, and don't reuse removed slots.
   // Add new features at the end of the enum.
 };

third_party/blink/renderer/core/feature_policy/document_policy_features.json5

@@ -10,7 +10,7 @@
     // which gets parsed from the header or the policy attribute.
     document_policy_name: {},
     value_name: {},
-    // value type allowed in mojom::PolicyValueType which is defined in 
+    // value type allowed in mojom::PolicyValueType which is defined in
     // third_party/blink/public/mojom/feature_policy/policy_value.mojom.
     value_type: {},
     // valid c++ expression strings, e.g. true/false, 1.0, -1.
@@ -46,5 +46,16 @@
       default_value: "max",
       depends_on: ["UnoptimizedImagePolicies"],
     },
+    {
+      // The ForceLoadAtTop policy lets pages opt-out of scrolling that
+      // automatically happens on page load. This includes fragment scrolls,
+      // text fragment scrolls (i.e. this provides an opt-out for the Scroll To
+      // Text feature), and scroll restoration.
+      name: "ForceLoadAtTop",
+      document_policy_name: "force-load-at-top",
+      value_name: "",
+      value_type: "Bool",
+      default_value: "false",
+    },
   ],
 }

third_party/blink/renderer/core/loader/document_loader.cc

@@ -181,6 +181,7 @@ DocumentLoader::DocumentLoader(
   frame_policy_ = params_->frame_policy.value_or(FramePolicy());
 
   document_policy_ = CreateDocumentPolicy();
+
   // If the document is blocked by document policy, there won't be content
   // in the sub-frametree, thus no need to initialize required_policy for
   // subtree.
@@ -1488,6 +1489,13 @@ void DocumentLoader::InstallNewDocument(
   if (!RuntimeEnabledFeatures::DocumentPolicyEnabled(owner_document))
     document_policy_ = DocumentPolicy::FeatureState{};
 
+  if (document_policy_.contains(
+          mojom::blink::DocumentPolicyFeature::kForceLoadAtTop)) {
+    navigation_scroll_allowed_ =
+        !(document_policy_[mojom::blink::DocumentPolicyFeature::kForceLoadAtTop]
+              .BoolValue());
+  }
+
   DocumentInit init =
       DocumentInit::Create()
           .WithDocumentLoader(this)

third_party/blink/renderer/core/loader/document_loader.h

@@ -321,6 +321,8 @@ class CORE_EXPORT DocumentLoader : public GarbageCollected<DocumentLoader>,
     return last_same_document_navigation_was_browser_initiated_;
   }
 
+  bool NavigationScrollAllowed() const { return navigation_scroll_allowed_; }
+
  protected:
   Vector<KURL> redirect_chain_;
 
@@ -551,6 +553,9 @@ class CORE_EXPORT DocumentLoader : public GarbageCollected<DocumentLoader>,
   // Whether this load request is a result of a browser initiated same-document
   // navigation.
   bool last_same_document_navigation_was_browser_initiated_ = false;
+
+  // Whether the document can be scrolled on load
+  bool navigation_scroll_allowed_ = true;
 };
 
 DECLARE_WEAK_IDENTIFIER_MAP(DocumentLoader);

third_party/blink/renderer/core/loader/frame_loader.cc

@@ -1183,7 +1183,8 @@ void FrameLoader::CommitDocumentLoader(
 void FrameLoader::RestoreScrollPositionAndViewState() {
   if (!frame_->GetPage() || !GetDocumentLoader() ||
       !GetDocumentLoader()->GetHistoryItem() ||
-      !GetDocumentLoader()->GetHistoryItem()->GetViewState()) {
+      !GetDocumentLoader()->GetHistoryItem()->GetViewState() ||
+      !GetDocumentLoader()->NavigationScrollAllowed()) {
     return;
   }
   RestoreScrollPositionAndViewState(
@@ -1307,12 +1308,15 @@ void FrameLoader::ProcessFragment(const KURL& url,
   // restoration type is manual, then we should not override it unless this
   // is a same document reload.
   bool should_scroll_to_fragment =
-      (load_start_type == kNavigationWithinSameDocument &&
-       !IsBackForwardLoadType(frame_load_type)) ||
-      (!GetDocumentLoader()->GetInitialScrollState().did_restore_from_history &&
-       !(GetDocumentLoader()->GetHistoryItem() &&
-         GetDocumentLoader()->GetHistoryItem()->ScrollRestorationType() ==
-             kScrollRestorationManual));
+      GetDocumentLoader()->NavigationScrollAllowed() &&
+      ((load_start_type == kNavigationWithinSameDocument &&
+        !IsBackForwardLoadType(frame_load_type)) ||
+       (!GetDocumentLoader()
+             ->GetInitialScrollState()
+             .did_restore_from_history &&
+        !(GetDocumentLoader()->GetHistoryItem() &&
+          GetDocumentLoader()->GetHistoryItem()->ScrollRestorationType() ==
+              kScrollRestorationManual)));
 
   view->ProcessUrlFragment(url,
                            load_start_type == kNavigationWithinSameDocument,