title: "Add Existing Item" feature disabled - how to enable / why does it happen?
body: <p>I didn't use MSVS since v2008 (but I never was good at it). The version I'm talking about is 2012. I work mostly in Emacs, but for the team I must create MSVS projects and maintain them in a workable state. Now once I create a file, MSVS would not let me add it to the solution (click on "Project" in "Solution Explorer" -> context menu -> select "Add" -> "Add Existing Item" is grayed out.)</p>
  
  <p>This used to work two days ago. Since then I've installed a plugin through NuGet that allows adding custom mime-types to IIS used in MSVS for ASP.NET projects. That was all the changes I've made.</p>
  
  <p>How do I troubleshoot this? Why can this possibly happen? What could possibly be the reason for disabling this functionality?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>Sorry, it looks like it isn't possible to do it while "debugging" the project - somehow I must have fatfingered a debug session w/o noticing it. When I restarted the Studio, it prompted me about the open debugging session. After restarting the Studio the functionality was back. Still, it's strange you can't add files during debugging... or maybe I'm missing something.</p>