Workaround JLine bug for mid-line tab completion

Before:
```
scala> {" ".char<TAB>}
charAt   chars

scala> {" ".char<CURSOR>}}
```

I noticed that pressing <SPACE>-<BACKSPACE> re-rendered the line
correctly, so I've added this workaround to our customization of
the JLine console reader.

After:

```
scala> {" ".char<TAB>}
charAt   chars

scala> {" ".char<CURSOR>}
```

We can delete this workaround when JLine 2.13.1 is released, but
I haven't heard back about when this might happen.

Author: retronym

src/repl-jline/scala/tools/nsc/interpreter/jline/JLineReader.scala

@@ -10,7 +10,8 @@ package scala.tools.nsc.interpreter.jline
 import java.util.{Collection => JCollection, List => JList}
 
 import _root_.jline.{console => jconsole}
-import jconsole.completer.{Completer, ArgumentCompleter}
+import jline.console.ConsoleReader
+import jline.console.completer.{CompletionHandler, Completer, ArgumentCompleter}
 import jconsole.history.{History => JHistory}
 
 
@@ -142,6 +143,27 @@ private class JLineConsoleReader extends jconsole.ConsoleReader with interpreter
       case NoCompletion       => ()
       case _                  => this addCompleter completer
     }
+
+    // This is a workaround for https://github.com/jline/jline2/issues/208
+    // and should not be necessary once we upgrade to JLine 2.13.1
+    ///
+    // Test by:
+    // scala> {" ".char}<LEFT><TAB>
+    //
+    // And checking we don't get an extra } on the line.
+    ///
+    val handler = getCompletionHandler
+    setCompletionHandler(new CompletionHandler {
+      override def complete(consoleReader: ConsoleReader, list: JList[CharSequence], i: Int): Boolean = {
+        try {
+          handler.complete(consoleReader, list, i)
+        } finally if (getCursorBuffer.cursor != getCursorBuffer.length()) {
+          print(" ")
+          getCursorBuffer.write(' ')
+          backspace()
+        }
+      }
+    })
     setAutoprintThreshold(400) // max completion candidates without warning
   }
 }