bug#29279: Sharing the margins

[martin rudalics <rudalics@gmx.at>]

 >>   > Yes.  But using margins from Emacs internals means that the
 >>   > window-parameters which hold the column specs will change behind the
 >>   > back of the Lisp applications, which I'm not sure is a Good Thing.
 >>
 >> I can see no harm in that.
 >
 > It's...unexpected.

Unless we are content with showing only the left part of the left margin
when a window gets too small as we do currently, we might also provide a
solution using "nominal" margin widths which apply when a window is wide
enough, and "realized" margin widths which apply when a window is too
narrow.  For line numbers, the nominal width could be somewhat
stretchable (as for the whiteroom, visual-fill-column cases) while most
others may have it fixed.  Probably, in addition an application may want
to specify that its realized margin width should be reset to zero
whenever the nominal width does not fit (line numbers probably want to
do that) or truncated to what is available.

Note that we earlier did shrink the margins when a window got too small
but did not re-enlarge them when the window was made wide again because
the nominal size was lost.  I eventually dropped that behavior and was
quite surprised that no-one protested.  But if we were to revive such
dynamical adjustment as sketched above, we'd have an unexpected "change
behind the back of the Lisp applications" as well.

martin