Fix issue #1131. I think this fix at least should substantially

improve matters. The intersection of the filter and the active being
empty means the diagnostics is not interesting.

src/Project.cpp

@@ -537,6 +537,8 @@ static String formatDiagnostics(const Diagnostics &diagnostics, Flags<QueryMessa
             } else {
                 filter = filter.intersected(active);
             }
+            if (filter.isEmpty())
+                return String();
         }
     }