GWT 2.7.0: Why I have to move my mouse in browser's client area to see actual loaded page?

Question

I'm developing a webapp prototype using GWT 2.7.0 and Vaadin-Polymer-Elements and I'm wondering about the page loading behaviour. The browser window remains white, until I move with my mouse cursor over it?! Even after packaging and deploying the app to an application server, this behaviour persists. Only if I have the chrome developer console open, the page shows up in milliseconds... To reproduce this, simply follow the GWT Polymer tutorial steps.

Any suggestions how to change this are appreciated.