init_iterator takes window

[Thien-Thi Nguyen <ttn@giblet.glug.org>]

rms,

per your suggestion, i have looked into using an iterator as a basis for
a current-column computation in the presence of variable-width fonts.  i
see that init_iterator takes a `struct window *' as its second arg and
from xdisp.c:1519 that this cannot be NULL.

is it possible for there to be two different windows open on the same
text with different display behaviors?  if so, which window to choose?
even if not, i'm confused about how to choose a window to to pass to
init_iterator; might there be a legitimate call to current-column where
there is no window available?

any clues appreciated.

thi