markdown-ts-mode: Don't enable unconditionally by default

* lisp/textmodes/markdown-ts-mode.el (markdown-ts-mode-maybe):
New function.
(auto-mode-alist): Bind ".md", ".markdown", and ".mdx" to
'markdown-ts-mode-maybe' instead of 'markdown-ts-mode'.
* etc/NEWS: Update the 'markdown-ts-mode' entry.

etc/NEWS

@@ -3938,19 +3938,12 @@ A major mode based on 'conf-mode' for editing ".npmrc" files.
 
 *** New major mode 'markdown-ts-mode'.
 A major mode based on the tree-sitter library for editing Markdown
-files.  This is now the default major mode for Markdown files.  If you
+files.  Markdown files are visited using this mode when the required
-don't have the necessary tree-sitter grammar libraries installed, or if
+tree-sitter grammars ('markdown' and 'markdown-inline') are available,
-your Emacs was built without tree-sitter support, Emacs will now show a
+or when the user has opted in via 'treesit-enabled-modes'.  Otherwise,
-warning to that effect when you visit a Markdown file.  If you don't
+Markdown files fall back to 'text-mode'.
-want to use this mode and want to avoid these warnings, add the
-following to your init file:
 
-  (add-to-list 'auto-mode-alist '("\\.md\\'" . fundamental-mode))
+To install the grammars, use 'M-x markdown-ts-mode-install-parsers'.
-  (add-to-list 'auto-mode-alist '("\\.markdown\\'" . fundamental-mode))
-  (add-to-list 'auto-mode-alist '("\\.mdx\\'" . fundamental-mode))
-
-This will cause Emacs to visit Markdown files in Fundamental mode, which
-was the default before this mode was added to Emacs.
 
 *** New major mode 'mhtml-ts-mode'.
 An optional major mode based on the tree-sitter library for editing HTML

lisp/textmodes/markdown-ts-mode.el

@@ -5401,14 +5401,14 @@ With a prefix argument, ARG, if needed, install parsers for `html',
     (cond ((treesit-ready-p '(markdown markdown-inline) t)
            (markdown-ts--set-up))
           (t
-           (warn "markdown-ts-mode cannot be set up; using fundamental-mode.
+           (warn "markdown-ts-mode cannot be set up; using text-mode.
 %s."
                  (if (treesit-available-p)
                      "The tree-sitter parsers `markdown' and `markdown-inline' were not found.
 Use the command `markdown-ts-mode-install-parsers' to install them.
 With a prefix argument, it can also install optional parsers"
                    "Emacs was built without Tree-sitter support, or could not load Tree-sitter"))
-           (fundamental-mode)))))
+           (text-mode)))))
 
 ;;;###autoload
 (define-derived-mode markdown-ts-mode text-mode "Markdown"
@@ -5619,11 +5619,25 @@ If non-nil and `point' is in a table, enable
          (remove-hook 'post-command-hook
                       #'markdown-ts--enable-in-table-mode 'local))))
 
+;;;###autoload
+(defun markdown-ts-mode-maybe ()
+  "Enable `markdown-ts-mode' when its grammars are available.
+Also propose to install the grammars when `treesit-enabled-modes'
+is t or contains the mode name."
+  (declare-function treesit-language-available-p "treesit.c")
+  (if (or (and (treesit-language-available-p 'markdown)
+               (treesit-language-available-p 'markdown-inline))
+          (eq treesit-enabled-modes t)
+          (memq 'markdown-ts-mode treesit-enabled-modes))
+      (markdown-ts-mode)
+    (text-mode)))
+
 ;;;###autoload
 (when (boundp 'treesit-major-mode-remap-alist)
-  (add-to-list 'auto-mode-alist '("\\.md\\'" . markdown-ts-mode))
+  (add-to-list 'auto-mode-alist '("\\.md\\'"        . markdown-ts-mode-maybe))
-  (add-to-list 'auto-mode-alist '("\\.markdown\\'" . markdown-ts-mode))
+  (add-to-list 'auto-mode-alist '("\\.markdown\\'"  . markdown-ts-mode-maybe))
-  (add-to-list 'auto-mode-alist '("\\.mdx\\'" . markdown-ts-mode))
+  (add-to-list 'auto-mode-alist '("\\.mdx\\'"       . markdown-ts-mode-maybe))
+  ;; To be able to toggle between an external package and core ts-mode:
   (add-to-list 'treesit-major-mode-remap-alist
                '(markdown-mode . markdown-ts-mode)))