Introduce and use minibuffer-mode. This fixes bug #47150

* lisp/minibuffer.el (minibuffer-mode): New derived mode.

* src/minibuf.c (syms_of_minibuf): New DEFSYMs Qminibuffer_mode,
Qminibuffer_inactive_mode, Qminibuffer_completing_file_name,
Qselect_frame_set_input_focus, Qadd_to_history.
(read_minibuf, set_minibuffer_mode, read_minibuf_unwind): Use the new DEFSYMs
in place of continual interning.
(set_minibuffer_mode): Put an active minibuffer into minibuffer-mode rather
than fundamental-mode.

* doc/emacs/mini.texi (Minibuffer Edit): Mention minibuffer-mode.

* doc/lispref/minibuf.texi (Intro to Minibuffers): Add a paragraph about
minibuffer-mode.

* etc/NEWS (Incompatible Lisp Changes in Emacs 28.1): Add an entry.

2 files changed, 11 insertions, 0 deletions

diff --git a/doc/emacs/mini.texi b/doc/emacs/mini.texi
index 1eba7074f76..03db6698fe5 100644
--- a/doc/emacs/mini.texi
+++ b/doc/emacs/mini.texi
@@ -247,6 +247,9 @@ You might need also to enable @code{minibuffer-depth-indicate-mode}
 to show the current recursion depth in the minibuffer prompt
 on recursive use of the minibuffer.
 
+  When active, the minibuffer is usually in @code{minibuffer-mode}.
+This is an internal Emacs mode without any special features.
+
 @findex minibuffer-inactive-mode
   When not active, the minibuffer is in @code{minibuffer-inactive-mode},
 and clicking @kbd{mouse-1} there shows the @file{*Messages*} buffer.
diff --git a/doc/lispref/minibuf.texi b/doc/lispref/minibuf.texi
index e922f1836b3..b8618199852 100644
--- a/doc/lispref/minibuf.texi
+++ b/doc/lispref/minibuf.texi
@@ -97,6 +97,14 @@ to be done.  @xref{Text from Minibuffer}, for the non-completion
 minibuffer local maps.  @xref{Completion Commands}, for the minibuffer
 local maps for completion.
 
+@cindex active minibuffer
+  An active minibuffer usually has major mode @code{minibuffer-mode}.
+This is an Emacs internal mode without any special features.  To
+customize the setup of minibuffers, we suggest you use
+@code{minibuffer-setup-hook} (@pxref{Minibuffer Misc}) rather than
+@code{minibuffer-mode-hook}, since the former is run later, after the
+minibuffer has been fully initialized.
+
 @cindex inactive minibuffer
   When a minibuffer is inactive, its major mode is
 @code{minibuffer-inactive-mode}, with keymap