Created attachment 258714 [details]
Gif demonstration of the issue
The Blur generation catches other windows into the blur generation when used in -webkit-backdrop-filter
How to reproduce:
I made a very simple example : http://mathieuhays.github.io/Backdrop-filter/
Open the inspector on a separate window.
Position it above the blurred part.
Toggle the css property -webkit-backdrop-filter off and on again.
Move the inspector out of the way.
You should see that the inspector window has been included in the blur rendering.
I can't reproduce on Yosemite. What OS, hardware and WebKit revision were you seeing this on?
I tried on both iMac and macbook on OS X 10.10.5
Webkit version 8.0.8 (10600.8.9, r188304)
I noticed that the issue also happens when you refresh the page with the inspector focused and on top of the blurred part.
I just tested again in the latest Safari version and it's now fixed