If two instances of VS are open, the Diff All window may open in wrong instance

[moudrick]

Steps to reproduce.

• open instance of VS2017 with solution
• open another instance with another solution
• try Diff All FIles in this second solution
• EXPECTED: diff result window in second instance.
  ACTUAL: diff result in first instance with first solution

[moudrick]

Just to give more information here, since it is probably the same issue.
Sometimes if I have open both VS 2017 and VS 2015, I press Diff All in VS2017, but it sends the diff result to VS 2015. In the past, I experiences the situation when even with the only open VS2017, it sometimes even opened my installed VS2015 for showing the diff result, but seems now it is no more the issue.