> Is window-point set to 1 as a side effect of making the temporary
 > buffer?

No.  It happens because I used BEG instead of the buffer's point.

 >>                                           Rather _you_ did set the old
 >> buffer's window point to 1 and it stays put there when you set w->buffer
 >> to old_buffer upon exiting `vertical-motion'.
 >
 > Do you have a suggestion how to reset point?

I attached I patch.  It compiles but is hardly trustworthy.  For
example, old_buffer might have got killed in the meantime.

 > /* If select_window is called with inhibit_point_swap non-zero it will
 >    not store point of the old selected window's buffer back into that
 >    window's pointm slot.  This is needed by Fset_window_configuration to
 >    avoid that the display routine is called with selected_window set to
 >    Qnil causing a subsequent crash.  */
 >
 > However, when I set a conditional breakpoint inhibit_point_swap!=0 this
 > did not interrupt execution, whereas with breakpoint select_window,
 > execution interrupts with inhibit_point_swap == 0, so I guess I don't
 > understand the comment.

IIRC this part is only relevant when you restore a window configuration,
for example, when leaving a `save-window-excursion'.  But I wouldn't
believe my own comments.

martin