fixes #4468

Author: retailcoder

Rubberduck.Core/AutoComplete/Service/SelfClosingPairHandler.cs

@@ -83,11 +83,13 @@ private bool HandleInternal(AutoCompleteEventArgs e, CodeString original, SelfCl
                 result = result.ReplaceLine(result.CaretPosition.StartLine, currentLine.TrimEnd());
             }
 
-            if (pair.OpeningChar == '(' && e.Character != '\b' &&
+            if (pair.OpeningChar == '(' && 
+                e.Character == pair.OpeningChar &&
                 !result.CaretLine.EndsWith($"{pair.OpeningChar}{pair.ClosingChar}"))
             {
                 // VBE eats it. just bail out.
-                return true;
+                result = null;
+                return false;
             }
 
             return true;

RubberduckTests/AutoComplete/SelfClosingPairHandlerTests.cs

@@ -62,5 +62,16 @@ public void GivenValidInput_InvokesSCP()
             Assert.IsTrue(Run(original, original, input, rePrettified, out var result));
             Assert.IsNotNull(result);
         }
+
+        [Test]
+        public void GivenOpeningParenthesisOnOtherwiseNonEmptyLine_ReturnsFalse()
+        {
+            var input = '(';
+            var original = "foo = DateSerial(Year|)".ToCodeString();
+            var rePrettified = "foo = DateSerial(Year(|))".ToCodeString();
+
+            Assert.IsFalse(Run(original, original, input, rePrettified, out var result));
+            Assert.IsNull(result);
+        }
     }
 }