Eglot: make markup invisible instead of deleting it (bug#79552)

We use gfm-view-mode to render Markdown before we hand over the string
to ElDoc (which usually put it in a 'special' mode "*eldoc*" buffer).
'gfm-view-mode' adds keymap text properties to make links clickable.  It
also makes some of the markup invisible with a special 'invisible'
property value which is specific to 'gfm-view-mode'.  We used to delete
the latter, therefore breaking the link-clicking.  Simply resetting the
regions with non-nil 'invisible' to 't' instead of deleting them fixes
this.  See also https://github.com/joaotavora/eglot/discussions/1238

* lisp/progmodes/eglot.el: Make invisible markup invisible
instead of deleting it.

* etc/EGLOT-NEWS: Mention bugfix.

Co-authored-by: João Távora <joaotavora@gmail.com>

2 files changed, 14 insertions, 3 deletions

diff --git a/etc/EGLOT-NEWS b/etc/EGLOT-NEWS
index 6505a6d8567..23104edd5d6 100644
--- a/etc/EGLOT-NEWS
+++ b/etc/EGLOT-NEWS
@@ -54,6 +54,12 @@ by the LSP server pertain.  This helps in skipping useless or harmful
 updates, avoiding flakiness with code actions and flickering overlays
 when the buffer is changed.
 
+** Markdown links migrating to *eldoc* buffer now clickable (bug#79552)
+
+Eglot now preserves crucial properties in the Markdown documentation
+provided by the LSP server, fixing a longstanding bug with clickable
+hyperlinks.  See also github#1238.
+
 
 * Changes in Eglot 1.18 (20/1/2025)
 
diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index b35d5e15e6c..6a7edef08b3 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -2018,7 +2018,7 @@ Doubles as an indicator of snippet support."
            (unless (bound-and-true-p yas-minor-mode) (yas-minor-mode 1))
            (apply #'yas-expand-snippet args)))))
 
- (defun eglot--format-markup (markup &optional mode)
+(defun eglot--format-markup (markup &optional mode)
   "Format MARKUP according to LSP's spec.
 MARKUP is either an LSP MarkedString or MarkupContent object."
   (let (string render-mode language)
@@ -2050,9 +2050,14 @@ MARKUP is either an LSP MarkedString or MarkupContent object."
         (goto-char (point-min))
         (let ((inhibit-read-only t))
           (when (fboundp 'text-property-search-forward)
+            ;; If `render-mode' is `gfm-view-mode', the `invisible'
+            ;; regions are set to `markdown-markup'.  Set them to 't'
+            ;; instead, since this has actual meaning in the "*eldoc*"
+            ;; buffer where we're taking this string (#bug79552).
             (while (setq match (text-property-search-forward 'invisible))
-              (delete-region (prop-match-beginning match)
-                             (prop-match-end match)))))
+              (put-text-property (prop-match-beginning match)
+                                 (prop-match-end match)
+                                 'invisible t))))
         (string-trim (buffer-string))))))
 
 (defun eglot--read-server (prompt &optional dont-if-just-the-one)