Limit to one popup, re-focus on additional notifications.

app/scripts/lib/notifications.js

@@ -21,12 +21,23 @@ function createMsgNotification (state) {
 }
 
 function showNotification() {
-  extension.windows.create({
-    url: 'notification.html',
-    type: 'detached_panel',
-    focused: true,
-    width: 360,
-    height: 500,
+  extension.windows.getAll({}, (windows) => {
+
+    let popupWindow = windows.find((win) => {
+      return win.type === 'popup'
+    })
+
+    if (popupWindow) {
+      return extension.windows.update(popupWindow.id, { focused: true })
+    }
+
+    extension.windows.create({
+      url: 'notification.html',
+      type: 'detached_panel',
+      focused: true,
+      width: 360,
+      height: 500,
+    })
   })
 }