Steps to replicate: I load up BlueJ 2.5.3 (or Greenfoot 1.5.5) and open up a code window. In the class I type out:
private final int MY_INT_CONSTANT = 0;
Until I press return, those underscores don't show up, so it looks exactly like I have typed a space, i.e. it appears as:
private final int MY INT CONSTANT = 0;
Which was a little disorienting. When I do press return (or scroll far enough down and back up that a refresh is triggered), the underscores appear, and stay. However, when I delete that text, the underscores (and just the underscores) seem to sometimes live on by themselves until the next refresh. I'm guessing the rendering is somehow not drawing things below the baseline properly. This behaviour is on Ubuntu Jaunty with the following java -version:
java version "1.6.0_0"
OpenJDK Runtime Environment (IcedTea6 1.4.1) (6b14-1.4.1-0ubuntu11)
OpenJDK Client VM (build 14.0-b08, mixed mode, sharing)