bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

Hi Phil,

Thanks for your detailed feedback.

> I can cause it to glitch by visiting a large file and leaning on the 
> page down key.  After a little while it fails to widen, and I'm left in 
> some narrowed portion of the buffer until I use a command which 
> re-triggers it.

That probably means you have a very big frame, you should set 
auto-narrow-display-length to a larger value than its default one (30000).

> I see an explicit list of supported commands in the code though, and I 
> worry about that.  Surely(?) we can't insist that all commands (if not 
> functions generally?) which need to deal with the full buffer (including 
> all the custom code out there in users' configs) should be updated to 
> make them explicitly aware of this new mode?  That doesn't seem 
> maintainable to me, and I can only imagine it causing lots of problems.

This new mode isn't necessary for ordinary files, so it will be activated 
only rarely.  That already means that it cannot cause "lots of problems". 
I think it's okay if it "breaks" a few commands, especially if the fix is 
only to add these commands to a list or to put a property on them.

> So-long is conservative in when it triggers by comparison, partly to be 
> confident of not causing unwanted issues, and partly by necessity 
> because its default action (a major mode) is very disruptive.

I see, so-long restricts its action to so-long-target-modes, is that 
correct?  I'm not sure such a restriction is necessary for 
auto-narrow-mode, it does not change anything apart from turning 
font-lock-mode off (by default) and narrowing the buffer.