1 files changed, 204 insertions, 0 deletions
diff --git a/browser/modules/NewTabPagePreloading.sys.mjs b/browser/modules/NewTabPagePreloading.sys.mjs
new file mode 100644
index 0000000000..bf6b050195
--- /dev/null
+++ b/browser/modules/NewTabPagePreloading.sys.mjs
@@ -0,0 +1,204 @@
+/* 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/. */
+
+/*
+ * This module is in charge of preloading 'new tab' pages for use when
+ * the user opens a new tab.
+ */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AboutNewTab: "resource:///modules/AboutNewTab.sys.mjs",
+  BrowserWindowTracker: "resource:///modules/BrowserWindowTracker.sys.mjs",
+  E10SUtils: "resource://gre/modules/E10SUtils.sys.mjs",
+  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+});
+
+export let NewTabPagePreloading = {
+  // Maximum number of instances of a given page we'll preload at any time.
+  // Because we preload about:newtab for normal windows, and about:privatebrowsing
+  // for private ones, we could have 3 of each.
+  MAX_COUNT: 3,
+
+  // How many preloaded tabs we have, across all windows, for the private and non-private
+  // case:
+  browserCounts: {
+    normal: 0,
+    private: 0,
+  },
+
+  get enabled() {
+    return (
+      this.prefEnabled &&
+      this.newTabEnabled &&
+      !lazy.AboutNewTab.newTabURLOverridden
+    );
+  },
+
+  /**
+   * Create a browser in the right process type.
+   */
+  _createBrowser(win) {
+    const {
+      gBrowser,
+      gMultiProcessBrowser,
+      gFissionBrowser,
+      BROWSER_NEW_TAB_URL,
+    } = win;
+
+    let oa = lazy.E10SUtils.predictOriginAttributes({ window: win });
+
+    let remoteType = lazy.E10SUtils.getRemoteTypeForURI(
+      BROWSER_NEW_TAB_URL,
+      gMultiProcessBrowser,
+      gFissionBrowser,
+      lazy.E10SUtils.DEFAULT_REMOTE_TYPE,
+      null,
+      oa
+    );
+    let browser = gBrowser.createBrowser({
+      isPreloadBrowser: true,
+      remoteType,
+    });
+    gBrowser.preloadedBrowser = browser;
+
+    let panel = gBrowser.getPanel(browser);
+    gBrowser.tabpanels.appendChild(panel);
+
+    return browser;
+  },
+
+  /**
+   * Move the contents of a preload browser across to a different window.
+   */
+  _adoptBrowserFromOtherWindow(window) {
+    let winPrivate = lazy.PrivateBrowsingUtils.isWindowPrivate(window);
+    // Grab the least-recently-focused window with a preloaded browser:
+    let oldWin = lazy.BrowserWindowTracker.orderedWindows
+      .filter(w => {
+        return (
+          winPrivate == lazy.PrivateBrowsingUtils.isWindowPrivate(w) &&
+          w.gBrowser &&
+          w.gBrowser.preloadedBrowser
+        );
+      })
+      .pop();
+    if (!oldWin) {
+      return null;
+    }
+    // Don't call getPreloadedBrowser because it'll consume the browser:
+    let oldBrowser = oldWin.gBrowser.preloadedBrowser;
+    oldWin.gBrowser.preloadedBrowser = null;
+
+    let newBrowser = this._createBrowser(window);
+
+    oldBrowser.swapBrowsers(newBrowser);
+
+    newBrowser.permanentKey = oldBrowser.permanentKey;
+
+    oldWin.gBrowser.getPanel(oldBrowser).remove();
+    return newBrowser;
+  },
+
+  maybeCreatePreloadedBrowser(window) {
+    // If we're not enabled, have already got one, are in a popup window, or the
+    // window is minimized / occluded, don't bother creating a preload browser -
+    // there's no point.
+    if (
+      !this.enabled ||
+      window.gBrowser.preloadedBrowser ||
+      !window.toolbar.visible ||
+      window.document.hidden
+    ) {
+      return;
+    }
+
+    // Don't bother creating a preload browser if we're not in the top set of windows:
+    let windowPrivate = lazy.PrivateBrowsingUtils.isWindowPrivate(window);
+    let countKey = windowPrivate ? "private" : "normal";
+    let topWindows = lazy.BrowserWindowTracker.orderedWindows.filter(
+      w => lazy.PrivateBrowsingUtils.isWindowPrivate(w) == windowPrivate
+    );
+    if (topWindows.indexOf(window) >= this.MAX_COUNT) {
+      return;
+    }
+
+    // If we're in the top set of windows, and we already have enough preloaded
+    // tabs, don't create yet another one, just steal an existing one:
+    if (this.browserCounts[countKey] >= this.MAX_COUNT) {
+      let browser = this._adoptBrowserFromOtherWindow(window);
+      // We can potentially get null here if we couldn't actually find another
+      // browser to adopt from. This can be the case when there's a mix of
+      // private and non-private windows, for instance.
+      if (browser) {
+        return;
+      }
+    }
+
+    let browser = this._createBrowser(window);
+    browser.loadURI(Services.io.newURI(window.BROWSER_NEW_TAB_URL), {
+      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+    });
+    browser.docShellIsActive = false;
+    browser._urlbarFocused = true;
+
+    // Make sure the preloaded browser is loaded with desired zoom level
+    let tabURI = Services.io.newURI(window.BROWSER_NEW_TAB_URL);
+    window.FullZoom.onLocationChange(tabURI, false, browser);
+
+    this.browserCounts[countKey]++;
+  },
+
+  getPreloadedBrowser(window) {
+    if (!this.enabled) {
+      return null;
+    }
+
+    // The preloaded browser might be null.
+    let browser = window.gBrowser.preloadedBrowser;
+
+    // Consume the browser.
+    window.gBrowser.preloadedBrowser = null;
+
+    // Attach the nsIFormFillController now that we know the browser
+    // will be used. If we do that before and the preloaded browser
+    // won't be consumed until shutdown then we leak a docShell.
+    // Also, we do not need to take care of attaching nsIFormFillControllers
+    // in the case that the browser is remote, as remote browsers take
+    // care of that themselves.
+    if (browser) {
+      let countKey = lazy.PrivateBrowsingUtils.isWindowPrivate(window)
+        ? "private"
+        : "normal";
+      this.browserCounts[countKey]--;
+      browser.removeAttribute("preloadedState");
+      browser.setAttribute("autocompletepopup", "PopupAutoComplete");
+    }
+
+    return browser;
+  },
+
+  removePreloadedBrowser(window) {
+    let browser = this.getPreloadedBrowser(window);
+    if (browser) {
+      window.gBrowser.getPanel(browser).remove();
+    }
+  },
+};
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  NewTabPagePreloading,
+  "prefEnabled",
+  "browser.newtab.preload",
+  true
+);
+XPCOMUtils.defineLazyPreferenceGetter(
+  NewTabPagePreloading,
+  "newTabEnabled",
+  "browser.newtabpage.enabled",
+  true
+);