To solve this extra window bug in Yosemite? I have got this kind of glossy window on top of other windows regularly in OSX Yosemite 10.10.2, two times per month approximately. 
You cannot press anything there. 
It blocks the user interface completely. 
To restart the system fixes the problem temporarily. 
The window stays there although you switch a program. 



I do not understand the cause of it, but I have produced it in Macbook Air 2013-mid and the newest Macbook Air so probably the problem is in the software. 

I do not know if some programs can interact for this bug. 
Used applications at the time of appearance of such a bug


Preview
Texpad
Microsoft Outlook 2013
Google Chrome
Finder
sometimes Terminal
Mail passively open sometimes
Dropbox (in continuous overflow, never managing perfect syncing)
BitTorrent Sync (however, this bug appeared much before this program so not causing the problem)


How can you solve this bug in Yosemite?
I filled a bug report to Apple to solve this buffer corruption problem, made here. The only workaround is to log out of the Mac and then log back in. That typically clears the graphics frame buffers. If not, a reboot will recover from the failure, but not prevent it from happening again.