Bug 1628476: Add keyboard navigation pref to allow focus mode changes on macOS r=Jamie,fluent-reviewers,settings-reviewers,flod,mconley

Differential Revision: https://phabricator.services.mozilla.com/D182266

browser/components/preferences/main.js

@@ -129,6 +129,7 @@ Preferences.addAll([
   { id: "general.smoothScroll", type: "bool" },
   { id: "widget.gtk.overlay-scrollbars.enabled", type: "bool", inverted: true },
   { id: "layout.spellcheckDefault", type: "int" },
+  { id: "accessibility.tabfocus", type: "int" },
 
   {
     id: "browser.preferences.defaultPerformanceSettings.enabled",
@@ -725,6 +726,14 @@ var gMainPane = {
       document.getElementById("defaultFont"),
       element => FontBuilder.readFontSelection(element)
     );
+    Preferences.addSyncFromPrefListener(
+      document.getElementById("useFullKeyboardNavigation"),
+      () => this.readUseFullKeyboardNavigation()
+    );
+    Preferences.addSyncToPrefListener(
+      document.getElementById("useFullKeyboardNavigation"),
+      () => this.writeUseFullKeyboardNavigation()
+    );
     Preferences.addSyncFromPrefListener(
       document.getElementById("checkSpelling"),
       () => this.readCheckSpelling()
@@ -2181,6 +2190,50 @@ var gMainPane = {
     migrationWizardDialog.showModal();
   },
 
+  /**
+   * Stores the original value of the tabfocus preference to enable proper
+   * restoration if unchanged (since we're mapping an int pref onto a checkbox).
+   */
+  _storedFullKeyboardNavigation: Preferences.get("accessibility.tabfocus"),
+
+  /**
+   * Returns true if any full keyboard nav is enabled and false otherwise, caching
+   * the current value to enable proper pref restoration if the checkbox is
+   * never changed.
+   *
+   * accessibility.tabfocus
+   * - an integer controlling the focusability of:
+   *     1  text controls
+   *     2  form elements
+   *     4  links
+   *     7  all of the above
+   */
+  readUseFullKeyboardNavigation() {
+    var pref = Preferences.get("accessibility.tabfocus");
+    this._storedFullKeyboardNavigation = pref.value;
+
+    return pref.value == 7;
+  },
+
+  /**
+   * Returns the value of the full keyboard nav preference represented by UI,
+   * preserving the preference's "hidden" value if the preference is
+   * unchanged and represents a value not strictly allowed in UI.
+   */
+  writeUseFullKeyboardNavigation() {
+    var checkbox = document.getElementById("useFullKeyboardNavigation");
+    if (checkbox.checked) {
+      return 7;
+    }
+    if (this._storedFullKeyboardNavigation != 7) {
+      // 1/2/4 values set via about:config should persist
+      return this._storedFullKeyboardNavigation;
+    }
+    // When the checkbox is unchecked, this pref shouldn't exist
+    // at all.
+    return undefined;
+  },
+
   /**
    * Stores the original value of the spellchecking preference to enable proper
    * restoration if unchanged (since we're mapping a tristate onto a checkbox).