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

app/scripts/lib/notifications.js

@@ -21,6 +21,16 @@ function createMsgNotification (state) {
 }
 
 function showNotification() {
+  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',
@@ -28,5 +38,6 @@ function showNotification() {
       width: 360,
       height: 500,
     })
+  })
 }