don't be smart, just calculate the common prefix based on lookup strings and insert it (IDEA-63646,IDEA-70503)

java/java-tests/testData/codeInsight/completion/normal/SaxParserCommonPrefix.java

@@ -0,0 +1,5 @@
+class Bar {
+  {
+    SaxP<caret>
+  }
+}

java/java-tests/testData/codeInsight/completion/normal/SaxParserCommonPrefix_after.java

@@ -0,0 +1,5 @@
+class Bar {
+  {
+    SAXParse<caret>
+  }
+}