Use heuristic for calculating completion prefix not only in plain text files but in other similar languages

platform/lang-impl/src/com/intellij/codeInsight/completion/CompletionData.java

@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2014 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 
 package com.intellij.codeInsight.completion;
 
@@ -177,7 +163,7 @@ public class CompletionData {
     final String prefix = getReferencePrefix(insertedElement, offsetInFile);
     if (prefix != null) return prefix;
 
-    if (insertedElement instanceof PsiPlainText || insertedElement instanceof PsiComment) {
+    if (insertedElement.getTextRange().equals(insertedElement.getContainingFile().getTextRange()) || insertedElement instanceof PsiComment) {
       return CompletionUtil.findJavaIdentifierPrefix(insertedElement, offsetInFile);
     }