desktop-presence: Use system presence data from electron-bridge.

Combined with work in the desktop app, this makes it possible for the
desktop app to clearly indicate to other users whether the current
user is active on the system and thus would see a desktop
notification, not just whether they are active in the current Zulip
window.

Essentially rewritten by tabbott to add unit tests and consider the
desktop app data authoritative.

frontend_tests/node_tests/activity.js

@@ -894,3 +894,26 @@ run_test('away_status', () => {
     activity.on_revoke_away(alice.user_id);
     assert(!user_status.is_away(alice.user_id));
 });
+
+run_test('electron_bridge', () => {
+    activity.client_is_active = false;
+    window.electron_bridge = undefined;
+    assert.equal(activity.compute_active_status(), activity.IDLE);
+
+    activity.client_is_active = true;
+    assert.equal(activity.compute_active_status(), activity.ACTIVE);
+
+    window.electron_bridge = {
+        idle_on_system: true,
+    };
+    assert.equal(activity.compute_active_status(), activity.IDLE);
+    activity.client_is_active = false;
+    assert.equal(activity.compute_active_status(), activity.IDLE);
+
+    window.electron_bridge = {
+        idle_on_system: false,
+    };
+    assert.equal(activity.compute_active_status(), activity.ACTIVE);
+    activity.client_is_active = true;
+    assert.equal(activity.compute_active_status(), activity.ACTIVE);
+});

static/js/activity.js

@@ -262,23 +262,42 @@ exports.update_huddles = function () {
     show_huddles();
 };
 
+exports.compute_active_status = function () {
+    // The overall algorithm intent for the `status` field is to send
+    // `ACTIVE` (aka green circle) if we know the user is at their
+    // computer, and IDLE (aka orange circle) if the user might not
+    // be:
+    //
+    // * For the webapp, we just know whether this window has focus.
+    // * For the electron desktop app, we also know whether the
+    //   user is active or idle elsewhere on their system.
+    //
+    // The check for `idle_on_system === undefined` is feature
+    // detection; older desktop app releases never set that property.
+    if (window.electron_bridge !== undefined
+            && window.electron_bridge.idle_on_system !== undefined) {
+        if (window.electron_bridge.idle_on_system) {
+            return exports.IDLE;
+        }
+        return exports.ACTIVE;
+    }
+
+    if (exports.client_is_active) {
+        return exports.ACTIVE;
+    }
+    return exports.IDLE;
+};
+
 function send_presence_to_server(want_redraw) {
-    var status;
     if (reload_state.is_in_progress()) {
         blueslip.log("Skipping querying presence because reload in progress");
         return;
     }
 
-    if (exports.client_is_active) {
-        status = exports.ACTIVE;
-    } else {
-        status = exports.IDLE;
-    }
-
     channel.post({
         url: '/json/users/me/presence',
         data: {
-            status: status,
+            status: exports.compute_active_status(),
             ping_only: !want_redraw,
             new_user_input: exports.new_user_input,
         },