Misaligned highlight on DoIt debugging

Hi,

I was trying to wrap my head around the Y combinator in trunk, update
10568. Of course I got it wrong, and the debugger opened up. Notice in
the attached image how the highlighted text - showing that code which is
currently executing - doesn't line up with syntactic boundaries.

The topmost frame also doesn't highlight correctly, but the first
(interesting) frame - UndefinedObject>>DoIt - does (highlighting the
block in  "y value: [...]".

Now clearly the debugger's showing some pretty-printed text, but what's
strange is that the first frame highlights correctly, but the subsequent
ones don't. I had expected that if pretty printing knocked the
highlighting out of alignment, that would be true for _all_ frames.

frank