Bug 1555753 - Always enable Browsing Context preservation in fission windows, r=farre

Differential Revision: https://phabricator.services.mozilla.com/D33544

--HG--
extra : moz-landing-system : lando

docshell/base/BrowsingContext.cpp

@@ -827,9 +827,7 @@ void BrowsingContext::Transaction::Apply(BrowsingContext* aBrowsingContext,
 }
 
 BrowsingContext::IPCInitializer BrowsingContext::GetIPCInitializer() {
-  MOZ_ASSERT(!mozilla::Preferences::GetBool(
-                 "fission.preserve_browsing_contexts", false) ||
-             IsContent());
+  // FIXME: We should assert that we're loaded in-content here. (bug 1553804)
 
   IPCInitializer init;
   init.mId = Id();

dom/base/nsFrameLoaderOwner.cpp

@@ -14,6 +14,9 @@
 #include "mozilla/dom/FrameLoaderBinding.h"
 #include "mozilla/dom/MozFrameLoaderOwnerBinding.h"
 
+using namespace mozilla;
+using namespace mozilla::dom;
+
 already_AddRefed<nsFrameLoader> nsFrameLoaderOwner::GetFrameLoader() {
   return do_AddRef(mFrameLoader);
 }
@@ -30,6 +33,35 @@ nsFrameLoaderOwner::GetBrowsingContext() {
   return nullptr;
 }
 
+bool nsFrameLoaderOwner::UseRemoteSubframes() {
+  RefPtr<Element> owner = do_QueryObject(this);
+  MOZ_ASSERT(this);
+
+  nsILoadContext* loadContext = owner->OwnerDoc()->GetLoadContext();
+  MOZ_DIAGNOSTIC_ASSERT(loadContext);
+
+  return loadContext->UseRemoteSubframes();
+}
+
+bool nsFrameLoaderOwner::ShouldPreserveBrowsingContext(
+    const mozilla::dom::RemotenessOptions& aOptions) {
+  if (aOptions.mReplaceBrowsingContext) {
+    return false;
+  }
+
+  // Don't preserve contexts if this is a chrome (parent process) window
+  // that is changing from remote to local.
+  if (XRE_IsParentProcess() && (!aOptions.mRemoteType.WasPassed() ||
+                                aOptions.mRemoteType.Value().IsVoid())) {
+    return false;
+  }
+
+  // We will preserve our browsing context if either fission is enabled, or the
+  // `preserve_browsing_contexts` pref is active.
+  return UseRemoteSubframes() ||
+         StaticPrefs::fission_preserve_browsing_contexts();
+}
+
 void nsFrameLoaderOwner::ChangeRemoteness(
     const mozilla::dom::RemotenessOptions& aOptions, mozilla::ErrorResult& rv) {
   RefPtr<mozilla::dom::BrowsingContext> bc;
@@ -37,18 +69,7 @@ void nsFrameLoaderOwner::ChangeRemoteness(
   // If we already have a Frameloader, destroy it, possibly preserving its
   // browsing context.
   if (mFrameLoader) {
-    // Don't preserve contexts if this is a chrome (parent process) window that
-    // is changing from remote to local.
-    bool isChromeRemoteToLocal =
-        XRE_IsParentProcess() && (!aOptions.mRemoteType.WasPassed() ||
-                                  aOptions.mRemoteType.Value().IsVoid());
-
-    // If this is a process switch due to a difference in Cross Origin Opener
-    // Policy, do not preserve the browsing context. Otherwise, save off the
-    // browsing context and use it when creating our new FrameLoader.
-    if (!aOptions.mReplaceBrowsingContext && !isChromeRemoteToLocal &&
-        mozilla::Preferences::GetBool("fission.preserve_browsing_contexts",
-                                      false)) {
+    if (ShouldPreserveBrowsingContext(aOptions)) {
       bc = mFrameLoader->GetBrowsingContext();
       mFrameLoader->SkipBrowsingContextDetach();
     }

dom/base/nsFrameLoaderOwner.h

@@ -52,6 +52,11 @@ class nsFrameLoaderOwner : public nsISupports {
   void ChangeRemoteness(const mozilla::dom::RemotenessOptions& aOptions,
                         mozilla::ErrorResult& rv);
 
+ private:
+  bool UseRemoteSubframes();
+  bool ShouldPreserveBrowsingContext(
+      const mozilla::dom::RemotenessOptions& aOptions);
+
  protected:
   virtual ~nsFrameLoaderOwner() = default;
   RefPtr<nsFrameLoader> mFrameLoader;

modules/libpref/init/StaticPrefList.h

@@ -2262,6 +2262,19 @@ VARCACHE_PREF(
   bool, false
 )
 
+//---------------------------------------------------------------------------
+// Prefs starting with "fission."
+//---------------------------------------------------------------------------
+
+// This pref has no effect within fission windows, it only controls the
+// behaviour within non-fission windows.
+VARCACHE_PREF(
+  Live,
+  "fission.preserve_browsing_contexts",
+  fission_preserve_browsing_contexts,
+  bool, false
+)
+
 //---------------------------------------------------------------------------
 // Prefs starting with "full-screen-api."
 //---------------------------------------------------------------------------