The attached file shows a reproduction recipe for a line-motion 
bug that had been nagging at me for some time.  The file also 
includes some debugging information and a preliminary diagnosis.

I don't know this area of the code very well.  While I can think 
of possible ways to fix this, each potential fix I've thought of 
so far raises questions that I don't feel secure about answering. 
If someone who knows this area well can make The Right Fix 
quickly, then great.  If not, then I would be happy to keep 
studying and make a patch for review.

Best regards,
-Karl