Conditional partial-width-window line truncation

[Davis Herring]

Often, on sufficiently large displays, it is useful to split an Emacs
frame vertically; particularly when reading source files that are
(generally) restrained to 80 columns, it would be a waste of space to do
the more usual horizontal split.  However, this interacts badly with
`truncate-partial-width-windows': in general this variable is a great
convenience, but in the case of 160-column (approximately) frames split
into two standard-width windows it is counterintuitive as the lines that 
were wrapped before are now truncated in what is still a "full-width 
window" as far as the user is concerned.

Might I suggest a new variable `truncate-maximum-window-width' which would 
specify an upper limit on the width of windows which should be 
automatically truncated when they are not a full frame wide?  The obvious 
default value would be 79, although perhaps as low as 69 would be useful.

I am unfortunately an Emacs Lisp hacker but not an Emacs' C hacker, so 
this suggestion is all I can give on the subject.

Thanks,
Davis Herring

PS - If this should wait until we're back on feature-addition mode, should
I re-submit it then, or what?

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