Steps to reproduce:
1) Go to http://www.apple.com/iphone/
2) Drag the tab out of the Safari window
The mini-window should contain a snapshot of the tab's contents.
The mini-window is blank.
I believe this also causes pixel test results of compositing tests to be blank on Windows. This blocks testing the accelerated part of <https://bugs.webkit.org/show_bug.cgi?id=77382>.
*** This bug has been marked as a duplicate of bug 52578 ***