Thanks Stephen for having tested at your side.

> (1) I see # jump whenever the mouse pointer is moved off the ruler while
>     holding down mouse-2, regardless of which direction you drag in
>     ...

  Yes, (1) is the same for me.

> (2) If you don't set the left margin, I see # jump as follows:
>     - If you drag from # straight down (due south), # jumps three
>       columns to the left. 
>     - If you drag from # southeast by one character, # jumps two columns
>       to the left.
>     - If you drag from # southeast by two characters, # jumps one column
>       to the left.
>     - If you drag from # southeast by more than two characters, # does
>       not jump to the left.
>     - If you drag from # southwest, # slides (but doesn't jump) to
>       whatever column you stop the drag on.  Moreover, once you drag #
>       southwest, you can slide # anywhere while holding mouse-2 with the
>       mouse pointer in the text area, just like when it is on the rule.
>

  I can't replicate any of the cases in (2); I always get 1 column hop.

> (3) If you set the left margin, the the number of columns jumped by
>     dragging due south equals (margin width + 3), if I'm not mistaken.

  In my setup I see the indicator jumps left for (margin width + 1)
columns. I'm afraid some other window parameters may be involved,
for example:

  - if you have a leftside fringe (I have it)
  - if you have a leftside scrollbar (I only have it rightside)
  - (maybe?) if your window manager sets thick window borders
    (I have just 3-4 pixel borders)

  I wrote a (quite rough) patch that seems to resolve all "hop"
problems on my installation. May I please ask if anyone has time
and patience to try the patch and report if it provides some benefit.