vertical-motion fails when tab in line

[David Reitter]

There's a bug in `vertical-motion' which will cause it to move the  
point not to the beginning of the next (visual) line when called with  
argument 1, but just a few characters to the right in the current  
line. This occurs iff there is a Tab in the line, as illustrated by  
the two screenshots attached.

The first screenshot shows the point at the leftmost character in the  
second visual line of a wrapped line. (could be a few characters to  
the right.)

Then we call (vertical-motion 1). The point doesn't move to where it  
should (see screenshot 2).

Another call to (vertical-motion 1) finally moves the cursor to the  
right position.

To reproduce, take a scratch buffer, enter a few lines of text wich  
will be wrapped and contain one (or more?) tabs. Place cursor near  
the end of a wrapped line, but not too far towards the end, and call  
M-: (vertical-motion 1).

Tried this in Aquamacs (based on very recent GNU CVS) and as well  
with a vanilla CVS build (Carbon port on OS X) with a fixed-width font.