Who can tell me what is the strange behavior of shadowBlur in html5 canvas?

Question

JavaScript

Subject:
jsfiddle.net/fr33z3/Rg9WD/2/
Actually, the problem is that when you first open the page, shadowBlur behaves like this:

After the refresh, everything renders well, but the bug appears under any conditions. document.onload doesn't help.
After refresh:

Answer 1

Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.22 (KHTML, like Gecko) Chrome/25.0.1364.152 Safari/537.22- everything is fine.

Answer 2

And what browser? I suggest enabling / disabling hardware acceleration, if it helps, the problem is in the video drivers.