From 775bd4b631e9303c20e4ebddd179960276065448 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <jp@neverwas.me>
Date: Wed, 27 Dec 2023 18:44:29 -0800
Subject: [PATCH] Sideline implied invisible-intangible coupling in ERC

* etc/ERC-NEWS: Add entry explaining removal of automatic `intangible'
propertizing of t-valued `invisible' messages.
* lisp/erc/erc.el (erc--insert-invisible-as-intangible-p): New flag
variable, a temporary escape hatch to regain pre-5.6 behavior
involving the modification of certain `invisible' messages.
(erc--insert-line): Gate unfavorable behavior behind
`erc--insert-invisible-as-intangible-p' flag.  Add comment clarifying
deferred initialization of `insert-position', which was part of the
many changes introduced as part of bug#60936.
---
 etc/ERC-NEWS    | 11 +++++++++++
 lisp/erc/erc.el | 27 ++++++++++++++++++++++-----
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index c883f575c15..6dc8af3c514 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -438,6 +438,17 @@ those folded onto the next line.  Such inconsistency made stamp
 detection overly complex and produced uneven results when toggling
 stamp visibility.
 
+*** Invisible message insertions not automatically made 'intangible'.
+Previously, when 'erc-display-message' and friends spotted the
+'invisible' text property with a value of t anywhere in text to be
+inserted, it would apply that property to the entire message, along
+with a t-valued 'intangible' property.  Beginning with ERC 5.6, users
+expecting this behavior will have to instead perform the treatment
+themselves.  To help with the transition, a temporary escape hatch has
+been made available to regain this behavior, but its existence is only
+guaranteed for this one minor version alone.  See source code in the
+vicinity of 'erc-insert-line' for more.
+
 *** Date stamps have become independent messages.
 ERC now inserts "date stamps" generated from the option
 'erc-timestamp-format-left' as separate, standalone messages.  This
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 02bfda143bc..f6962910da0 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -3282,6 +3282,21 @@ If END is a marker, possibly update its position."
   (unless (eq end erc-insert-marker)
     (set-marker end nil)))
 
+(defvar erc--insert-invisible-as-intangible-p nil
+  "When non-nil, ensure certain invisible messages are also intangible.
+That is, single out any message inserted via `erc-insert-line'
+that lacks a trailing newline but has a t-valued `invisible'
+property anywhere along its length, and ensure it's both
+`invisible' t and `intangible' t throughout.  Note that this is
+merely an escape hatch for accessing aberrant pre-5.6 behavior
+that ERC considers a bug because it applies a practice described
+as obsolete in the manual, and it does so heavy-handedly.  That
+the old behavior only acted when the input lacked a trailing
+newline was likely accidental but is ultimately incidental.  See
+info node `(elisp) Special Properties' for specifics.  Beware
+that this flag and the behavior it restores may disappear at any
+time, so if you need them, please let ERC know with \\[erc-bug].")
+
 (defvar erc--insert-line-function nil
   "When non-nil, an alterntive to `insert' for inserting messages.")
 
@@ -3310,13 +3325,15 @@ preformatted or anticipated by third-party members of the various
 modification hooks)."
   (when string
     (with-current-buffer (or buffer (process-buffer erc-server-process))
-      (let ((insert-position (marker-position erc-insert-marker)))
-        (let ((string string) ;; FIXME! Can this be removed?
-              (buffer-undo-list t)
+      (let (insert-position)
+        ;; Initialize ^ below to thwart rogue `erc-insert-pre-hook'
+        ;; members that dare to modify the buffer's length.
+        (let ((buffer-undo-list t)
               (inhibit-read-only t))
-          (unless (string-match "\n$" string)
+          (unless (string-suffix-p "\n" string)
             (setq string (concat string "\n"))
-            (when (erc-string-invisible-p string)
+            (when (and erc--insert-invisible-as-intangible-p
+                       (erc-string-invisible-p string))
               (erc-put-text-properties 0 (length string)
                                        '(invisible intangible) string)))
           (erc-log (concat "erc-display-message: " string
-- 
2.39.2