Fix outlining banner with whitespace between hash and region.

Fixes #4105.

src/EditorFeatures/CSharp/Outlining/Outliners/RegionDirectiveOutliner.cs

@@ -15,7 +15,7 @@ internal class RegionDirectiveOutliner : AbstractSyntaxNodeOutliner<RegionDirect
         private static string GetBannerText(DirectiveTriviaSyntax simpleDirective)
         {
             var kw = simpleDirective.DirectiveNameToken;
-            var prefixLength = simpleDirective.HashToken.Span.Length + kw.Span.Length;
+            var prefixLength = kw.Span.End - simpleDirective.Span.Start;
             var text = simpleDirective.ToString().Substring(prefixLength).Trim();
 
             if (text.Length == 0)

src/EditorFeatures/CSharpTest/Outlining/RegionDirectiveOutlinerTests.cs

@@ -87,6 +87,22 @@ static void Main(string[] args)
     }
     #endregion|]
 }
+");
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.Outlining)]
+        [WorkItem(4105, "https://github.com/dotnet/roslyn/issues/4105")]
+        public void SpacesBetweenPoundAndRegionShouldNotAffectBanner()
+        {
+            TestRegion("Region", @"
+class C
+{
+[|#  region R$$egion
+    static void Main(string[] args)
+    {
+    }
+#  endregion|]
+}
 ");
         }
     }