paper-button shadow

[novotnyj]

Raised paper-button sometimes has this shadow:

Sometimes this happens after clicking button. But it allways happens when I click button, go to different tab in browser and then go back to tab with paper-button. Seems like button keeps being focused.
I am using latest Ubuntu with latest Chrome.

[molnara]

I have the exact problem. Windows 7, both Chrome 40 and IE 11. I happens on alt-tab as well and the button has to have focus for to bug out.