This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
Try calling moveToFront() on one of the JInternalFrames. If that doesn't work, the solution might be to make another JInternalFrame for your scrolling area, and then resize that as appropriate. Not ideal, but it might be the only way.