Merge pull request #3478 from minrk/rewarn

restore "unsaved changes" warning on unload

IPython/html/static/notebook/js/notebook.js

@@ -357,9 +357,15 b' var IPython = (function (IPython) {'
             if (kill_kernel) {
                 that.kernel.kill();
             }
-            // if we are autosaving, trigger an autosave on nav-away
-            if (that.dirty && that.autosave_interval && ! that.read_only) {
-                that.save_notebook();
+            // if we are autosaving, trigger an autosave on nav-away.
+            // still warn, because if we don't the autosave may fail.
+            if (that.dirty && ! that.read_only) {
+                if ( that.autosave_interval ) {
+                    that.save_notebook();
+                    return "Autosave in progress, latest changes may be lost.";
+                } else {
+                    return "Unsaved changes will be lost.";
+                }
             };
             // Null is the *only* return value that will make the browser not
             // pop up the "don't leave" dialog.