Environment hangs, completely at times, or just for several seconds. If it resumes after hanging, you are able to close the tab where this .SQL file would show up.
In any case, you cannot see or edit anything in there, seeing this Visual Studio 2010 branded blue background all over the contents area.
The solution, which worked for me - get the DVD with Visual Studio installation files and install from
WCU\DAC\DACProjectSystemSetup_enu.msi with default options.
According to stackoverflow, this problem may be caused by installing SQL Server 2008 R2 after installing VS 2010.
Somebody there pointed out that if you have SP1, you should use an SP1 DVD, but I just used the retail ISO, no problems so far.