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.

2 files changed, 14 insertions, 3 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