bug#29279: Sharing the margins

[Dmitry Gutov <dgutov@yandex.ru>]

On 11/16/17 5:50 PM, Eli Zaretskii wrote:

>> Sounds straightforward to me. Since we know the sizes of all the columns
>> in advance, we can just substract them from the target total width, and
>> pad with the resulting number of spaces.
> 
> Maybe, but IME things are rarely so easy.

In that case, I think we've made sure they are easy (by requiring all 
columns' widths to be known in advance).

>> Further, even though we have a separate accessor for its width
>> (line-number-display-width), if a package depends on it and needs to
>> draw something based on its value, it should want to be notified when
>> there is a change (*). window-configuration-change-hook seems natural.
>> Unless we have a separate hook for that?
> 
> The way this feature is designed and implemented, it doesn't lend
> itself easily to hooks, primarily because it works in the inner-most
> level of redisplay.

Then maybe using the margins for it will be a necessary price, with the 
corresponding performance hit (though hopefully not), just to enable the 
extensibility our users are accustomed to.

>> Can't you save the necessary data to a variable, finish redisplay,
>> and then run the hook (if the data says so)?
> 
> That would be pointless, because there are already hooks which work
> before redisplay or after it finishes.  All such a hook needs to do is
> compare the value returned by line-number-display-width with the last
> value it saw.  That's what I did in tabulated-list-mode, which has
> some unique requirements in this area.  Avoiding the comparison
> doesn't justify a new hook.

Hmm, I'm not sure it would be as pointless as you say: normally, it's 
most important to be notified of some change _eventually_, and not 
necessarily during some process such as redisplay. It would at least 
save the user the problem of puzzling out how to do this, and what to 
compare.

On the other hand, it could be the argument for margin changes not to 
run the usual hooks, because any sane called could compare margin widths 
before and after.

> And anyway, what do you envision that a hook function will want to do?
> Most probably, it will want to change the window dimensions, or affect
> what's on display in some other way, which means an immediate second
> redisplay cycle.

Affect what's on display, yes, most likely.

> So we gain nothing by making the display engine call
> the hook.

Yeah. I was suggesting to call the hook later, though.

>> It's somewhat hypothetical, but I'd like to refer to (*) above. That is,
>> somebody will probably ask for that anyway, sooner or later.
> 
> Somebody already did, and I declined for now, because I think the same
> effect can be achieved via existing hooks.

Do you have margin-using examples that likewise couldn't be "achieved 
via existing hooks" if margin changes didn't call 
window-configuration-change-hook?