Update app.js

Oops! Accidentally trigger refresh or bookmark.

src/app.js

@@ -227,6 +227,10 @@ Please make a backup of your contracts and start using http://remix.ethereum.org
   if (window.location.protocol.indexOf('https') === 0) {
     toolTip('You are using an `https` connection. Please switch to `http` if you are using Remix against an `http Web3 provider` or allow Mixed Content in your browser.')
   }
+  // Oops! Accidentally trigger refresh or bookmark.
+  window.onbeforeunload = function () {
+    return 'Are you sure you want to leave?'
+  }
 
   function importExternal (url, cb) {
     handleImports.import(url,