Open Radar

 
I've tried it on multiple hardware configurations, and got the same result, but here are two of those HW configs : 

First : 
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MacBook Pro (Retina, 13-inch, Mid 2014) 
Processor: 3 GHz Intel Core i7 
Memory: 16 GB 1600 MHz DDR3 
Graphics: Intel Iris 1536 MB 
Serial Number: C02NR1FRG3QR 
OSX 10.10.1 (14B25) 

Second : 
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MacBook Pro (Retina, 15-inch, Late 2013) 
Processor: 2,3 GHz Intel Core i7 
Memory: 16 GB 1600 MHz DDR3 
Graphics: NVIDIA GeForce GT 750M 2048 MB 
Serial Number: C02MR49UFD57 
OSX 10.10.2

DESCRIPTION OF PROBLEM
On Yosemite the captured images of a selected window may contain rendering glitches caused by animations of floating windows. In our case the capturing method is CGWindowListCreateImageFromArray. 

Only those machines are affected where transparency is enabled, and only the transparent parts of the window, whose image we are capturing, may contain glitches. Let's call this window the target window. The target window needs to be placed underneath of a floating (always-on-top) window, which has animations from time-to-time, like the "CPU History" child window of "Activity Monitor". And as a side-effect of those animations the captured image of the target window may contain blurred parts of the floating window. 

Although only the floating window should not affect the captured images at all.

STEPS TO REPRODUCE
Sample project can be downloaded from here : https://www.cubbyusercontent.com/pl/Yosemite%20Blinking%20issue/_1976a2c0c790406c8990fee5d7157b38 . 

1. Make sure transparency is not disabled among Accessibility settings. 
2. Please see the attached demo application (called AlwaysOnTopBlurDemo) and start it. 
3. When the application starts the floating "Top window" is intentionally placed on top of the "Target window's" title bar. If the floating "Top window" is not above the title bar of the "Target window" for any reason, please relocate it so. 
4. There is a third window which belongs to this sample app, which is called "Image of target window". This window displays the last captured image of the target window. 

Expected result 
The captured images shown in the "Image of target window" should be either as you can see on the "Active target window" or the "Inactive target window" attached image. 

Actual result 
The displayed image in the "Image of target window" is usually like as the "Active target window" or the "Inactive target window" attached image, but about every 10th image is like the "Glitchy target window" attached image.