Assert foreground before checking file open

src/VisualStudio/Core/Def/Implementation/ProjectSystem/RunningDocumentTableEventTracker.cs

@@ -128,7 +128,11 @@ public int OnAfterDocumentWindowHide(uint docCookie, IVsWindowFrame pFrame)
         public int OnBeforeSave(uint docCookie)
             => VSConstants.E_NOTIMPL;
 
-        public bool IsFileOpen(string fileName) => _runningDocumentTable.IsFileOpen(fileName);
+        public bool IsFileOpen(string fileName)
+        {
+            _foregroundAffinitization.AssertIsForeground();
+            return _runningDocumentTable.IsFileOpen(fileName);
+        }
 
         /// <summary>
         /// Attempts to get a text buffer from the specified moniker.