Move some things around

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-# Make " Narrow" lighter customizable
-
-The " Narrow" string comes from `src/xdisp.c:decode_mode_spec`:
-
-``` c
-case 'n':
-  if (BUF_BEGV (b) > BUF_BEG (b) || BUF_ZV (b) < BUF_Z (b))
-    return " Narrow";
-```
-
-This is probably just a matter of returning the contents of a Lisp
-variable instead of this constant string.
-
-TODO:
-
-1. get the string value of a variable in C
-2. define a customizable string variable
-3. write a news entry
-4. write a patch
-5. extra credits: display string properties
-
-## Get the string value of a variable in C
-
-`decode_mode_spec` has some relevant snippets:
-
-- Given a `Lisp_Object obj`, `SSDATA(obj)` gives the string value as a
-  `char*`.
-
-- How to get a variable's `Lisp_Object`?
-    - `BVAR` works for buffer-local variables
-    - `V${lispname//-/_}`
-
-## Define a customizable string variable
-
-### Defining variables visible to C code
-
-The C macro `DEFVAR_LISP(string-name, field-name)` does the following:
-
-    define a static `Lisp_Objfwd` variable v
-    get the address of globals._f##field-name &f
-
-    defvar_lisp(v, string-name, &f)
-
-As explained in the comments above `DEFVAR_LISP`, `globals` is a
-global variable defined in `globals.h`, which is "auto-generated by
-make-docfile" and exposes fields, `#define`s and `Lisp_Object`s for
-every global variable.
-
-make-docfile (`lib-src/make-docfile.c`) takes C files as input and
-searches all occurences of `^ +DEFSYM[ \t(]`, `^ +DEFVAR_[ILB]` or
-`^DEFU`, analyses what comes after and generates appropriate
-definitions for `globals.h`.
-
-`defvar_lisp` allocates a symbol using `Fmake_symbol`.
-
-### Making it customizable
-
-`lisp/cus-start.el` defines customizable properties of symbols defined
-by C code.
-
-AFAICT, there is no need to assign the default value right after
-defining the variable with `DEFVAR_LISP`: e.g. `shell-file-name` is
-`DEFVAR_LISP`ed in `src/callproc.c` and its default value is set in…
-Mmm.  Not in `cus-start.el`.  There is this snippet in
-`callproc.c:init_callproc`:
-
-  ``` c
-sh = getenv ("SHELL");
-Vshell_file_name = build_string (sh ? sh : "/bin/sh");
-  ```
-
-But when starting with `SHELL=rofl emacs -Q`, Custom says that the
-value "has been changed outside Customize".  Changed from what to
-what?
-
-`cus-start.el` may contain a hint:
-
-``` elisp
-;; Elements of this list have the form:
-;; …
-;; REST is a set of :KEYWORD VALUE pairs.  Accepted :KEYWORDs are:
-;; :standard - standard value for SYMBOL (else use current value)
-;; …
-```
-
-Except that nope, this does not work.  Giving `:standard " Narrow"`
-and looking at the variable in Custom yields
-
-    narrow-lighter: nil
-        [State]: CHANGED outside Customize. (mismatch)
-
-A better example might be `overlay-arrow-string`, whose default value
-is set right after `DEFVAR_LISP` by calling `build_pure_c_string`.
-
-Why `build_pure_c_string` and not `build_string`?  From "(elisp) Pure
-Storage":
-
-> Emacs Lisp uses two kinds of storage for user-created Lisp objects:
-> “normal storage” and “pure storage”.  Normal storage is where all
-> the new data created during an Emacs session are kept (see Garbage
-> Collection).  Pure storage is used for certain data in the preloaded
-> standard Lisp files—data that should never change during actual use
-> of Emacs.
->
-> Pure storage is allocated only while ‘temacs’ is loading the
-> standard preloaded Lisp libraries.  In the file ‘emacs’, it is
-> marked as read-only (on operating systems that permit this), so that
-> the memory space can be shared by all the Emacs jobs running on the
-> machine at once.
-
-"(elisp) Building Emacs" explains that "temacs" is the minimal Elisp
-interpreter built by compiling all C files in `src/`; temacs then
-loads Elisp sources and creates the "emacs" executable by dumping its
-current state into a file.
-
-## Debug stuff
-
-### Unicode characters represented as octal sequences
-
-Trying to customize the new variable to any string with non-ASCII
-characters fails: they show up as sequences of backslash-octal codes.
-For some reason they show up fine in the Help and Custom buffers.
-
-Things to investigate:
-
-1. Should the `Lisp_Object` be created with something other than
-   `build_pure_c_string`? 🙅
-2. What does the code calling `decode_mode_spec` do with the returned
-   string? **🎉**
-3. (Does `SSDATA` make some transformation before returning the
-   string? 🤷)
-4. (Should a specialized Custom setter be defined? 🤷)
-
-#### Should the `Lisp_Object` be created with something other than    `build_pure_c_string`?
-
-Maybe this would work?
-
-``` c
-Vnarrow_lighter = make_multibyte_string(" Narrow", strlen(" Narrow"),
-                                        strlen(" Narrow)");
-```
-
-That looks too ugly though, let's try something else.
-
-Maybe `STRING_SET_MULTIBYTE(Vnarrow_lighter)` would help?
-
-*compiles and tries*
-
-… Nope, it does not.
-
-#### What does the code calling `decode_mode_spec` do with the returned string?
-
-``` c
-spec = decode_mode_spec (it->w, c, field, &string);
-multibyte = STRINGP (string) && STRING_MULTIBYTE (string);
-```
-
-*slowly turns around*
-
-*finds `string` standing right there with a blank stare*
-
-Gah!  How long have you been there?
-
-``` c
-/* Return a string for the output of a mode line %-spec for window W,
-   generated by character C.  […]  Return a Lisp string in
-   *STRING if the resulting string is taken from that Lisp string.
-   […] */
-static const char *
-decode_mode_spec (struct window *w, register int c, int field_width,
-                  Lisp_Object *string)
-{
-  Lisp_Object obj;
-  /* … */
-  obj = Qnil;
-  *string = Qnil;
-
-  switch (c)
-    {
-    /* … */
-    }
-
-  if (STRINGP (obj))
-    {
-      *string = obj;
-      return SSDATA (obj);
-    }
-  else
-    return "";
-}
-```
-
-Alright then:
-
-``` c
-case 'n':
-  if (BUF_BEGV (b) > BUF_BEG (b) || BUF_ZV (b) < BUF_Z (b))
-      obj = Vnarrow_lighter;
-  break;
-```
-
-### Why do string properties not show up?
-
-🤷
-
-## Extra credit
-
-Maybe it would be simpler to have the narrowing lighter work like the
-" Compiling" lighter (cf. `compilation-in-progress` variable), i.e. adding an entry to `minor-mode-alist`.