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diff --git a/devtools/server/actors/resources/parent-process-document-event.js b/devtools/server/actors/resources/parent-process-document-event.js
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+++ b/devtools/server/actors/resources/parent-process-document-event.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  TYPES: { DOCUMENT_EVENT },
+} = require("resource://devtools/server/actors/resources/index.js");
+const isEveryFrameTargetEnabled = Services.prefs.getBoolPref(
+  "devtools.every-frame-target.enabled",
+  false
+);
+const {
+  WILL_NAVIGATE_TIME_SHIFT,
+} = require("resource://devtools/server/actors/webconsole/listeners/document-events.js");
+
+class ParentProcessDocumentEventWatcher {
+  /**
+   * Start watching, from the parent process, for DOCUMENT_EVENT's "will-navigate" event related to a given Watcher Actor.
+   *
+   * All other DOCUMENT_EVENT events are implemented from another watcher class, running in the content process.
+   * Note that this other content process watcher will also emit one special edgecase of will-navigate
+   * retlated to the iframe dropdown menu.
+   *
+   * We have to move listen for navigation in the parent to better handle bfcache navigations
+   * and more generally all navigations which are initiated from the parent process.
+   * 'bfcacheInParent' feature enabled many types of navigations to be controlled from the parent process.
+   *
+   * This was especially important to have this implementation in the parent
+   * because the navigation event may be fired too late in the content process.
+   * Leading to will-navigate being emitted *after* the new target we navigate to is notified to the client.
+   *
+   * @param WatcherActor watcherActor
+   *        The watcher actor from which we should observe document event
+   * @param Object options
+   *        Dictionary object with following attributes:
+   *        - onAvailable: mandatory function
+   *          This will be called for each resource.
+   */
+  async watch(watcherActor, { onAvailable }) {
+    this.watcherActor = watcherActor;
+    this.onAvailable = onAvailable;
+
+    // List of listeners keyed by innerWindowId.
+    // Listeners are called as soon as we emitted the will-navigate
+    // resource for the related WindowGlobal.
+    this._onceWillNavigate = new Map();
+
+    // Filter browsing contexts to only have the top BrowsingContext of each tree of BrowsingContexts…
+    const topLevelBrowsingContexts = this.watcherActor
+      .getAllBrowsingContexts()
+      .filter(browsingContext => browsingContext.top == browsingContext);
+
+    // Only register one WebProgressListener per BrowsingContext tree.
+    // We will be notified about children BrowsingContext navigations/state changes via the top level BrowsingContextWebProgressListener,
+    // and BrowsingContextWebProgress.browsingContext attribute will be updated dynamically everytime
+    // we get notified about a child BrowsingContext.
+    // Note that regular web page toolbox will only have one BrowsingContext tree, for the given tab.
+    // But the Browser Toolbox will have many trees to listen to, one per top-level Window, and also one per tab,
+    // as tabs's BrowsingContext context aren't children of their top level window!
+    //
+    // Also save the WebProgress and not the BrowsingContext because `BrowsingContext.webProgress` will be undefined in destroy(),
+    // while it is still valuable to call `webProgress.removeProgressListener`. Otherwise events keeps being notified!!
+    this.webProgresses = topLevelBrowsingContexts.map(
+      browsingContext => browsingContext.webProgress
+    );
+    this.webProgresses.forEach(webProgress => {
+      webProgress.addProgressListener(
+        this,
+        Ci.nsIWebProgress.NOTIFY_STATE_DOCUMENT
+      );
+    });
+  }
+
+  /**
+   * Wait for the emission of will-navigate for a given WindowGlobal
+   *
+   * @param Number innerWindowId
+   *        WindowGlobal's id we want to track
+   * @return Promise
+   *         Resolves immediatly if the WindowGlobal isn't tracked by any target
+   *         -or- resolve later, once the WindowGlobal navigates to another document
+   *         and will-navigate has been emitted.
+   */
+  onceWillNavigateIsEmitted(innerWindowId) {
+    // Only delay the target-destroyed event if the target is for BrowsingContext for which we will emit will-navigate
+    const isTracked = this.webProgresses.find(
+      webProgress =>
+        webProgress.browsingContext.currentWindowGlobal.innerWindowId ==
+        innerWindowId
+    );
+    if (isTracked) {
+      return new Promise(resolve => {
+        this._onceWillNavigate.set(innerWindowId, resolve);
+      });
+    }
+    return Promise.resolve();
+  }
+
+  onStateChange(progress, request, flag, status) {
+    const isStart = flag & Ci.nsIWebProgressListener.STATE_START;
+    const isDocument = flag & Ci.nsIWebProgressListener.STATE_IS_DOCUMENT;
+    if (isDocument && isStart) {
+      const { browsingContext } = progress;
+      // Ignore navigation for same-process iframes when EFT is disabled
+      if (
+        !browsingContext.currentWindowGlobal.isProcessRoot &&
+        !isEveryFrameTargetEnabled
+      ) {
+        return;
+      }
+      // Ignore if we are still on the initial document,
+      // as that's the navigation from it (about:blank) to the actual first location.
+      // The target isn't created yet.
+      if (browsingContext.currentWindowGlobal.isInitialDocument) {
+        return;
+      }
+
+      // Only emit will-navigate for top-level targets.
+      if (
+        this.watcherActor.sessionContext.type == "all" &&
+        browsingContext.isContent
+      ) {
+        // Never emit will-navigate for content browsing contexts in the Browser Toolbox.
+        // They might verify `browsingContext.top == browsingContext` because of the chrome/content
+        // boundary, but they do not represent a top-level target for this DevTools session.
+        return;
+      }
+      const isTopLevel = browsingContext.top == browsingContext;
+      if (!isTopLevel) {
+        return;
+      }
+
+      const newURI = request instanceof Ci.nsIChannel ? request.URI.spec : null;
+      const { innerWindowId } = browsingContext.currentWindowGlobal;
+      this.onAvailable([
+        {
+          browsingContextID: browsingContext.id,
+          innerWindowId,
+          resourceType: DOCUMENT_EVENT,
+          name: "will-navigate",
+          time: Date.now() - WILL_NAVIGATE_TIME_SHIFT,
+          isFrameSwitching: false,
+          newURI,
+        },
+      ]);
+      const callback = this._onceWillNavigate.get(innerWindowId);
+      if (callback) {
+        this._onceWillNavigate.delete(innerWindowId);
+        callback();
+      }
+    }
+  }
+
+  get QueryInterface() {
+    return ChromeUtils.generateQI([
+      "nsIWebProgressListener",
+      "nsISupportsWeakReference",
+    ]);
+  }
+
+  destroy() {
+    this.webProgresses.forEach(webProgress => {
+      webProgress.removeProgressListener(
+        this,
+        Ci.nsIWebProgress.NOTIFY_STATE_DOCUMENT
+      );
+    });
+    this.webProgresses = null;
+  }
+}
+
+module.exports = ParentProcessDocumentEventWatcher;