In Ubuntu 8.04, mlterm 2.9.4. I have noticed this problem more and more often. Sometimes, in the output from a command like "ls", some lines in the output are "partly blank". Hitting that line with the mouse causes the characters to appear. If I refocus the window in any way, the lines appear. Alt-TAB to cycle across programs will cause the redraw.
I'm trying to make a photo for you to see this effect. Sometimes, I've noticed that using the keys causes the redraw to fix the problem. In the gimp, I find that hitting the screenshot tool is able to get a snapshot of the trouble, and then the terminal re-draws right away to fix the problem.
Sometimes, the output of commands does not show at the bottom of the terminal until I hit return a few times.
I'm attaching some pictures to show the problem.