Eglot: Simplify markdown rendering support (bug#81150)

Now that markdown-ts-view-mode is demoted to "experimental" in emacs-31,
simplify bits of eglot.el and rewrite docs to be more neutral.  In
practice 'gfm-view-mode' is still used if found, just like before, but
intrepid users can still try the "experimental" modes.

* lisp/progmodes/eglot.el (eglot-documentation-renderer): Rewrite
doc string.
(eglot--accepted-formats): Rewrite.
(eglot--builtin-mdown-p): Remove.

* doc/misc/eglot.texi (Customization Variables): Rewrite entry.

* etc/EGLOT-NEWS: Tweak.

doc/misc/eglot.texi

@@ -999,13 +999,14 @@ will consider it to be part of the workspace.  The default is
 @cindex markdown renderer
 @item eglot-documentation-renderer
 This variable controls how Eglot renders at-point documentation imported
-from the server (@pxref{Eglot Features}).  By default, the variable's
-value is set during startup to the @code{gfm-view-mode} markdown
-renderer if available.  This utility visually enhances the documentation
-content through fontification and other formatting.  If you set it to
-@code{t}, plain text will be requested from the server and no rendering
-is attempted.  If the variable's value is @code{nil}, Eglot will attempt
-to find a suitable renderer every time.
+from the server (@pxref{Eglot Features}).  The default value is
+@code{nil}, meaning Eglot selects a suitable Markdown renderer on each
+use---for example, @code{gfm-view-mode} from @code{markdown-mode} on
+NonGNU ELPA, which enhances documentation through fontification and
+other formatting.  You can also set it to always use a specific major mode symbol, such
+as the aforementioned @code{gfm-view-mode}, or the experimental
+@code{markdown-ts-view-mode}.  If you set it to @code{t}, plain text is
+requested from the server and no rendering is attempted.
 
 @item eglot-mode-map
 This variable is the keymap for binding Eglot-related command.  It is in

etc/EGLOT-NEWS

@@ -32,11 +32,14 @@ New key bindings: 'k' shuts down, 'r' reconnects, 'e' visits the events
 buffer, 'w' shows workspace configuration, and 'RET' invokes
 'eglot-describe-connection'.
 
-** 'eglot-documentation-renderer' replaces 'eglot-prefer-plaintext'.
+** New variable 'eglot-documentation-renderer' (bug#80127)
 
 The variable 'eglot-documentation-renderer' replaces the now-obsolete
-'eglot-prefer-plaintext'.  By default, the variable selects a markdown
-renderer to use throughout the session.
+'eglot-prefer-plaintext' and offers more control over what major mode is
+used to render markdown snippets.  By default, the variable is nil and
+'gfm-view-mode' from NonGNU ELPA's markdown-mode package is used if
+found.  You may also set it to the experimental 'markdown-ts-view-mode'
+in Emacs versions with tree-sitter markdown support.
 
 
 * Changes in Eglot 1.23 (2/4/2026)

lisp/progmodes/eglot.el

@@ -537,15 +537,12 @@ or file operation kinds not in the alist."
   "If non-nil, activate Eglot in cross-referenced non-project files."
   :type 'boolean)
 
-(defcustom eglot-documentation-renderer (cond ((fboundp 'gfm-view-mode)
-                                               'gfm-view-mode)
-                                              (t
-                                               nil))
-  "Control rendering of LSP documentation fragments.
-If set to the major mode symbol `gfm-view-mode', request
-markdown-snippets and use `gfm-view-mode' to render it.
-If t, always request and render plain text snippets.  If set to nil,
-decide dynamically."
+(defcustom eglot-documentation-renderer nil
+  "Controls rendering of LSP documentation fragments.
+If set to a major mode symbol like `gfm-view-mode', or the experimental
+`markdown-ts-view-mode', request markdown snippets and use that mode to
+render them.  If t, request and render plain text instead.  If nil,
+request markdown snippets and select a renderer dynamically."
   :type '(choice (const :tag "Plain text" t)
                 (const :tag "Auto-detect" nil)
                 (function :tag "Renderer"))
@@ -738,16 +735,11 @@ This can be useful when using docker to run a language server.")
 
 (declare-function treesit-grammar-location "treesit.c")
 
-(defun eglot--builtin-mdown-p ()
-  (and (fboundp 'markdown-ts-view-mode)
-       (fboundp 'treesit-grammar-location)
-       (treesit-grammar-location 'markdown)))
-
 (defun eglot--accepted-formats ()
-  (if (and (not (eq t eglot-documentation-renderer))
-           (or (fboundp 'gfm-view-mode) (eglot--builtin-mdown-p)))
-      ["markdown" "plaintext"]
-    ["plaintext"]))
+  (if (or (eq t eglot-documentation-renderer)
+          (not (or eglot-documentation-renderer (fboundp 'gfm-view-mode))))
+      ["plaintext"]
+    ["markdown" "plaintext"]))
 
 (defconst eglot--uri-path-allowed-chars
   (let ((vec (copy-sequence url-path-allowed-chars)))