Temporarily add the current position to the browser history when the viewer is idle

This patch attempts to address an issue in the old `PDFHistory` implementation, where the current position wouldn't be correctly saved when the browser was closed.
In theory this *should* already be working, however as the discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1153393 showed, it seems that both `pagehide` and `beforeunload` arrive to late to successfully update the history during closing.

Hence a timeout is used to *temporarily* add the current position to the browser history when the viewer is idle.
Note that we need to take care not to update the browser history too often, since that would render the viewer more or unusable. Furthermore, if the timeout is *too* long it may end up effectively disable this whole functionality.

The `UPDATE_VIEWAREA_TIMEOUT` constant is thus a heuristic value, which we may need to tweak taking the above into account.

web/pdf_history.js

@@ -13,11 +13,13 @@
  * limitations under the License.
  */
 
-import { parseQueryString, waitOnEventOrTimeout } from './ui_utils';
+import { cloneObj, parseQueryString, waitOnEventOrTimeout } from './ui_utils';
 import { getGlobalEventBus } from './dom_events';
 
 // Heuristic value used when force-resetting `this._blockHashChange`.
 const HASH_CHANGE_TIMEOUT = 1000; // milliseconds
+// Heuristic value used when adding a temporary position to the browser history.
+const UPDATE_VIEWAREA_TIMEOUT = 2000; // milliseconds
 
 /**
  * @typedef {Object} PDFHistoryOptions
@@ -117,8 +119,8 @@ class PDFHistory {
     // The browser history contains a valid entry, ensure that the history is
     // initialized correctly on PDF document load.
     let destination = state.destination;
-    this._updateInternalState(destination, state.uid);
-
+    this._updateInternalState(destination, state.uid,
+                              /* removeTemporary = */ true);
     if (destination.dest) {
       this.initialBookmark = JSON.stringify(destination.dest);
 
@@ -275,16 +277,25 @@ class PDFHistory {
   /**
    * @private
    */
-  _tryPushCurrentPosition() {
+  _tryPushCurrentPosition(temporary = false) {
     if (!this._position) {
       return;
     }
     let position = this._position;
+    if (temporary) {
+      position = cloneObj(this._position);
+      position.temporary = true;
+    }
 
     if (!this._destination) {
       this._pushOrReplaceState(position);
       return;
     }
+    if (this._destination.temporary) {
+      // Always replace a previous *temporary* position.
+      this._pushOrReplaceState(position, /* forceReplace = */ true);
+      return;
+    }
     if (this._destination.hash === position.hash) {
       return; // The current document position has not changed.
     }
@@ -337,7 +348,12 @@ class PDFHistory {
   /**
    * @private
    */
-  _updateInternalState(destination, uid) {
+  _updateInternalState(destination, uid, removeTemporary = false) {
+    if (removeTemporary && destination && destination.temporary) {
+      // When the `destination` comes from the browser history,
+      // we no longer treat it as a *temporary* position.
+      delete destination.temporary;
+    }
     this._destination = destination;
     this._currentUid = uid;
     this._uid = this._currentUid + 1;
@@ -347,6 +363,11 @@ class PDFHistory {
    * @private
    */
   _updateViewarea({ location, }) {
+    if (this._updateViewareaTimeout) {
+      clearTimeout(this._updateViewareaTimeout);
+      this._updateViewareaTimeout = null;
+    }
+
     this._position = {
       hash: this._isViewerInPresentationMode ?
         `page=${location.pageNumber}` : location.pdfOpenParams.substring(1),
@@ -357,6 +378,30 @@ class PDFHistory {
     if (this._popStateInProgress) {
       return;
     }
+
+    if (UPDATE_VIEWAREA_TIMEOUT > 0) {
+      // When closing the browser, a 'pagehide' event will be dispatched which
+      // *should* allow us to push the current position to the browser history.
+      // In practice, it seems that the event is arriving too late in order for
+      // the session history to be successfully updated.
+      // (For additional details, please refer to the discussion in
+      //  https://bugzilla.mozilla.org/show_bug.cgi?id=1153393.)
+      //
+      // To workaround this we attempt to *temporarily* add the current position
+      // to the browser history only when the viewer is *idle*,
+      // i.e. when scrolling and/or zooming does not occur.
+      //
+      // PLEASE NOTE: It's absolutely imperative that the browser history is
+      // *not* updated too often, since that would render the viewer more or
+      // less unusable. Hence the use of a timeout to delay the update until
+      // the viewer has been idle for `UPDATE_VIEWAREA_TIMEOUT` milliseconds.
+      this._updateViewareaTimeout = setTimeout(() => {
+        if (!this._popStateInProgress) {
+          this._tryPushCurrentPosition(/* temporary = */ true);
+        }
+        this._updateViewareaTimeout = null;
+      }, UPDATE_VIEWAREA_TIMEOUT);
+    }
   }
 
   /**
@@ -408,14 +453,23 @@ class PDFHistory {
 
     // This case corresponds to navigation backwards in the browser history.
     if (state.uid < this._currentUid && this._position && this._destination) {
-      if (this._destination.page &&
-          this._destination.page !== this._position.first &&
-          this._destination.page !== this._position.page) {
+      let shouldGoBack = false;
+
+      if (this._destination.temporary) {
+        // If the `this._destination` contains a *temporary* position, always
+        // push the `this._position` to the browser history before moving back.
+        this._pushOrReplaceState(this._position);
+        shouldGoBack = true;
+      } else if (this._destination.page &&
+                 this._destination.page !== this._position.first &&
+                 this._destination.page !== this._position.page) {
         // If the `page` of the `this._destination` is no longer visible,
         // push the `this._position` to the browser history before moving back.
         this._pushOrReplaceState(this._destination);
         this._pushOrReplaceState(this._position);
-
+        shouldGoBack = true;
+      }
+      if (shouldGoBack) {
         // After `window.history.back()`, we must not enter this block on the
         // resulting 'popstate' event, since that may cause an infinite loop.
         this._currentUid = state.uid;
@@ -427,8 +481,8 @@ class PDFHistory {
 
     // Navigate to the new destination.
     let destination = state.destination;
-    this._updateInternalState(destination, state.uid);
-
+    this._updateInternalState(destination, state.uid,
+                              /* removeTemporary = */ true);
     if (destination.dest) {
       this.linkService.navigateTo(destination.dest);
     } else if (destination.hash) {