Unexpected behaviour of bundle.js

When we load our browser we expect that bundle.js fire as soon as the browser renders UI but this is not happening as expected. The bundle.js runs only when we have either mouse events or key presses.

Note: All the steps below should be followed without a click on the window as it triggers off bundle.js

How to reproduce :
1.Open index.html.
2.Open Console(Ctrl+shift+I).
3.As soon as click the app window bundle.js starts running with the message.

Starting up...           bundle.js:7904
gui_startup from GUI...  bundle.js:7904

Expected: bundle.js runs after UI is painted.