From 5a690fc54fa58b0443ab8b6cb65e6588d5b5e9ec Mon Sep 17 00:00:00 2001
From: Henrik Lissner <git@henrik.io>
Date: Mon, 30 Jun 2025 20:54:21 +0200
Subject: [PATCH] feat(:term): confine undo to comint/eshell prompts

Otherwise, undo could delete entire chunks of a shell buffer. This
addresses the issue in eshell, comint shells, and derivatives (like
ielm, shell, or inferior-* shells).

Ref: #8410
---
 lisp/doom-ui.el               | 14 ++++++++++++++
 modules/term/eshell/config.el | 17 ++++++++---------
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/lisp/doom-ui.el b/lisp/doom-ui.el
index dbc83ae68..b93cfda64 100644
--- a/lisp/doom-ui.el
+++ b/lisp/doom-ui.el
@@ -350,6 +350,20 @@ windows, switch to `doom-fallback-buffer'. Otherwise, delegate to original
 (after! comint
   (setq-default comint-buffer-maximum-size 2048)  ; double the default
 
+  ;; UX: Temporarily disable undo history between command executions. Otherwise,
+  ;;   undo could destroy output while it's being printed or delete buffer
+  ;;   contents past the boundaries of the current prompt.
+  (add-hook 'comint-exec-hook #'buffer-disable-undo)
+  (defadvice! doom--comint-enable-undo-a (process _string)
+    :after #'comint-output-filter
+    (let ((start-marker comint-last-output-start))
+      (when (and (< start-marker
+                    (or (if process (process-mark process))
+                        (point-max-marker)))
+                 (eq (char-before start-marker) ?\n)) ;; Account for some of the IELM’s wilderness.
+        (buffer-enable-undo)
+        (setq buffer-undo-list nil))))
+
   ;; Protect prompts from accidental modifications.
   (setq-default comint-prompt-read-only t)
 
diff --git a/modules/term/eshell/config.el b/modules/term/eshell/config.el
index 00b0130a3..aafa0fcac 100644
--- a/modules/term/eshell/config.el
+++ b/modules/term/eshell/config.el
@@ -85,15 +85,14 @@ You should use `set-eshell-alias!' to change this.")
   (add-hook 'eshell-mode-hook #'+eshell-init-h)
   (add-hook 'eshell-exit-hook #'+eshell-cleanup-h)
 
-  ;; UX: Temporarily disable undo history between command executions. Undo can
-  ;;   destroy output while it's being printed to stdout.
-  (let (old-undo-list)
-    (add-hook! 'eshell-pre-command-hook
-      (setq old-undo-list buffer-undo-list
-            buffer-undo-list nil))
-    (add-hook! 'eshell-post-command-hook
-      (setq buffer-undo-list old-undo-list)
-      (clrhash undo-equiv-table)))
+  ;; UX: Temporarily disable undo history between command executions. Otherwise,
+  ;;   undo could destroy output while it's being printed or delete buffer
+  ;;   contents past the boundaries of the current prompt.
+  (add-hook 'eshell-pre-command-hook #'buffer-disable-undo)
+  (add-hook! 'eshell-post-command-hook
+    (defun +eshell--enable-undo-h ()
+      (buffer-enable-undo (current-buffer))
+      (setq buffer-undo-list nil)))
 
   ;; UX: Prior output in eshell buffers should be read-only. Otherwise, it's
   ;;   trivial to make edits in visual modes (like evil's or term's