Fix incompleteness in the implementation of minibuffer-follows-selected-frame

In particular, add a new value to the variable, and fix several bugs apparent
with the implementation up till now.

* doc/emacs/mini.texi (Basic Minibuffer): Add a description of the new
non-nil, non-t value of minibuffer-follows-selected-frame.

* doc/emacs/trouble.texi (Quitting): Add a description of how C-g handles
recursive minibuffers when typed in one which isn't the most nested.

* doc/lispref/minibuf.texi (Intro to Minibuffers): Add an @dfn for "active
minibuffer".
(Minibuffer Commands): Document that exit-minibuffer throws an error when not
invoked from the innermost Minibuffer.
(Recursive Mini): Amend the description of the visibility of outer level
minibuffers.
(Minibuffer Misc): In the description of the minibuffer hooks, replace "the
minibuffer" with "a minibuffer".

* etc/NEWS (Entry announcing minibuffer-follows-selected-frame): Add a
description of the new non-nil, non-t value.

* lisp/cus-start.el (top level): make the customize entry for
minibuffer-follows-selected-frame a choice between three entries.

* lisp/minibuffer.el (exit-minibuffer): throw an error when we're not in the
most nested minibuffer.
(top level): Bind C-g to abort-minibuffers in minibuffer-local-map.

* lisp/window.el (window-deletable-p): return the symbol `frame' when (amongst
other things) minibuffer-follows-selected-frame is t.

* src/eval.c (internal_catch): Add a mechanism to (throw 'exit t) repeatedly
when the throw currently being processed doesn't terminate the current
minibuffer.

* src/lisp.h (this_minibuffer_depth): New extern declaration
(minibuf_level): extern declaration moved here from window.h.

* src/minibuf.c (minibuffer_follows_frame, minibuf_stays_put)
(minibuf_moves_frame_when_opened): New and amended functions to query the
value of minibuffer-follows-selected-frame.
(choose_minibuf_frame): check (minibuf > 1) in place of (minibufer > 0) at a
particular place.  At another place, check that an alleged frame is so and is
live.  Before selecting a non-miniwindow on a different frame, ensure it
really is a different frame.
(move_minibuffer_onto_frame): Stack up all recursive minibuffers on the target
frame.  Check the minibuf_window isn't in the old frame before setting that
frame's miniwindow to an inactive minibuffer.
(Finnermost_minibuffer_p, Fabort_minibuffers): New primitives.
(this_minibuffer_depth): New function.
(read_minibuf): Record the calling frame in a variable, and switch back to it
after the recursive edit has terminated normally, using
select-frame-set-input-focus.  Stack up all the recursive minibuffers on the
miniwindow where a new minibuffer is being opened.  After the recursive edit,
switch the selected window away from the expired minibuffer's window.
(nth_minibuffer): New function.
(minibuffer-follows-selected-frame): Change from a DEFVAR_BOOL to a
DEFVAR_LISP.

* src/window.c (decode_next_window_args): Set *minibuf to w's mini-window's
content when that content is a minibuffer.

* src/window.h (minibuf_level) Declaration moved from here to lisp.h.

3 files changed, 29 insertions, 12 deletions

diff --git a/doc/emacs/mini.texi b/doc/emacs/mini.texi
index c7c8fb30ac6..f81e64bdf9b 100644
--- a/doc/emacs/mini.texi
+++ b/doc/emacs/mini.texi
@@ -76,9 +76,13 @@ default, the active minibuffer moves to this new frame.  If you set
 the user option @code{minibuffer-follows-selected-frame} to
 @code{nil}, then the minibuffer stays in the frame where you opened
 it, and you must switch back to that frame in order to complete (or
-abort) the current command.  Note that the effect of the command, when
-you finally finish using the minibuffer, always takes place in the
-frame where you first opened it.
+abort) the current command.  If you set that option to a value which
+is neither @code{nil} nor @code{t}, the minibuffer moves frame only
+after a recursive minibuffer has been opened in the current command
+(@pxref{Recursive Mini,,, elisp}).  This option is mainly to retain
+(approximately) the behavior prior to Emacs 28.1.  Note that the
+effect of the command, when you finally finish using the minibuffer,
+always takes place in the frame where you first opened it.
 
 @node Minibuffer File
 @section Minibuffers for File Names
diff --git a/doc/emacs/trouble.texi b/doc/emacs/trouble.texi
index 4da3d4a3e89..9a638818c91 100644
--- a/doc/emacs/trouble.texi
+++ b/doc/emacs/trouble.texi
@@ -57,6 +57,13 @@ incremental search, @kbd{C-g} behaves specially; it may take two
 successive @kbd{C-g} characters to get out of a search.
 @xref{Incremental Search}, for details.
 
+  If you type @kbd{C-g} in a minibuffer, this quits the command that
+opened that minibuffer, closing it.  If that minibuffer is not the
+most recently opened one (which can happen when
+@code{minibuffer-follows-selected-frame} is @code{nil} (@pxref{Basic
+Minibuffer})), @kbd{C-g} also closes the more recently opened ones,
+quitting their associated commands, after asking you for confirmation.
+
   On MS-DOS, the character @kbd{C-@key{Break}} serves as a quit character
 like @kbd{C-g}.  The reason is that it is not feasible, on MS-DOS, to
 recognize @kbd{C-g} while a command is running, between interactions
diff --git a/doc/lispref/minibuf.texi b/doc/lispref/minibuf.texi
index f0036f0ccfc..d316c1f0602 100644
--- a/doc/lispref/minibuf.texi
+++ b/doc/lispref/minibuf.texi
@@ -82,10 +82,12 @@ there is an active minibuffer; such a minibuffer is called a
 incrementing the number at the end of the name.  (The names begin with
 a space so that they won't show up in normal buffer lists.)  Of
 several recursive minibuffers, the innermost (or most recently
-entered) is the active minibuffer.  We usually call this @emph{the}
-minibuffer.  You can permit or forbid recursive minibuffers by setting
-the variable @code{enable-recursive-minibuffers}, or by putting
-properties of that name on command symbols (@xref{Recursive Mini}.)
+entered) is the @dfn{active minibuffer}--it is the one you can
+terminate by typing @key{RET} (@code{exit-minibuffer}) in.  We usually
+call this @emph{the} minibuffer.  You can permit or forbid recursive
+minibuffers by setting the variable
+@code{enable-recursive-minibuffers}, or by putting properties of that
+name on command symbols (@xref{Recursive Mini}.)
 
   Like other buffers, a minibuffer uses a local keymap
 (@pxref{Keymaps}) to specify special key bindings.  The function that
@@ -2380,7 +2382,8 @@ minibuffer.
 
 @deffn Command exit-minibuffer
 This command exits the active minibuffer.  It is normally bound to
-keys in minibuffer local keymaps.
+keys in minibuffer local keymaps.  The command throws an error if the
+current buffer is not the active minibuffer.
 @end deffn
 
 @deffn Command self-insert-and-exit
@@ -2594,8 +2597,11 @@ returns zero.
 If this variable is non-@code{nil}, you can invoke commands (such as
 @code{find-file}) that use minibuffers even while the minibuffer is
 active.  Such invocation produces a recursive editing level for a new
-minibuffer.  The outer-level minibuffer is invisible while you are
-editing the inner one.
+minibuffer.  By default, the outer-level minibuffer is invisible while
+you are editing the inner one.  If you have
+@code{minibuffer-follows-selected-frame} set to @code{nil}, you can
+have minibuffers visible on several frames at the same time.
+@xref{Basic Minibuffer,,, emacs}.
 
 If this variable is @code{nil}, you cannot invoke minibuffer commands
 when the minibuffer is active, not even if you switch to another window
@@ -2623,7 +2629,7 @@ active minibuffer.
 @end defun
 
 @defvar minibuffer-setup-hook
-This is a normal hook that is run whenever the minibuffer is entered.
+This is a normal hook that is run whenever a minibuffer is entered.
 @xref{Hooks}.
 @end defvar
 
@@ -2641,7 +2647,7 @@ called once, for the outermost use of the minibuffer.
 @end defmac
 
 @defvar minibuffer-exit-hook
-This is a normal hook that is run whenever the minibuffer is exited.
+This is a normal hook that is run whenever a minibuffer is exited.
 @xref{Hooks}.
 @end defvar