From 0a6daa141220a99bae591ae5935a43b7710254bd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20T=C3=A1vora?= <joaotavora@gmail.com>
Date: Thu, 25 Dec 2025 16:40:03 +0000
Subject: [PATCH] Eglot: speed up symbol highlighting (bug#80072)

In large buffers, responses to textDocument/documentHighlight
requests may return thousands of occurrences.  Avoid incurring
expensive eglot--lsp-position-to-point calls, by restricting the
highlights to visible region in the active window.

* lisp/progmodes/eglot.el (eglot-highlight-eldoc-function):
Rework.
---
 lisp/progmodes/eglot.el | 34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index 415444d0c47..96f1c7e9820 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -3936,19 +3936,27 @@ for which LSP on-type-formatting should be requested."
        :success-fn
        (lambda (highlights)
          (mapc #'delete-overlay eglot--highlights)
-         (setq eglot--highlights
-               (eglot--when-buffer-window buf
-                 (mapcar
-                  (eglot--lambda ((DocumentHighlight) range)
-                    (pcase-let ((`(,beg . ,end)
-                                 (eglot-range-region range)))
-                      (let ((ov (make-overlay beg end)))
-                        (overlay-put ov 'face 'eglot-highlight-symbol-face)
-                        (overlay-put ov 'eglot--overlay t)
-                        (overlay-put ov 'modification-hooks
-                                     `(,(lambda (o &rest _) (delete-overlay o))))
-                        ov)))
-                  highlights))))
+         (setq eglot--highlights nil)
+         (eglot--when-buffer-window buf
+           ;; Don't highlight occurrences that aren't
+           ;; visible. (bug#80072).
+           (let* ((w (car (get-buffer-window-list)))
+                  (ws (window-start w)) (we (window-end w))
+                  (ls (1- (line-number-at-pos ws t)))
+                  (le (1- (line-number-at-pos we t))))
+             (mapc
+              (eglot--lambda ((DocumentHighlight) range)
+                (when-let* ((l (cl-getf (cl-getf range :start) :line))
+                            (_ (and (>= l ls) (<= l le))))
+                  (pcase-let ((`(,beg . ,end)
+                               (eglot-range-region range)))
+                    (let ((ov (make-overlay beg end)))
+                      (overlay-put ov 'face 'eglot-highlight-symbol-face)
+                      (overlay-put ov 'eglot--overlay t)
+                      (overlay-put ov 'modification-hooks
+                                   `(,(lambda (o &rest _) (delete-overlay o))))
+                      (push ov eglot--highlights)))))
+              highlights))))
        :hint :textDocument/documentHighlight)
       nil)))