Subject: bug#60936: 30.0.50; ERC >5.5: Add erc-fill style based on visual-line-mode

From: "J.P." <[email-masked]>
Date: Tue, 18 Jul 2023 06:55:31 -0700

Quick fixup (misc/test-custom-opts just caught some sloppiness in my
Custom :type specs).

>From 828db2d91b0f47f8a758e3011bb3cbf817168564 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Fri, 14 Jul 2023 06:12:30 -0700
Subject: [PATCH] [5.6] Make erc-fill-wrap work with left-hand stamps

* etc/ERC-NEWS: Remove all mention of option `erc-timestamp-align-to'
supporting a value of `margin', which has been removed.
* lisp/erc/erc-backend.el (erc--reveal-prompt, erc--conceal-prompt):
New generic functions with default implementations factored out from
`erc--unhide-prompt' and `erc--hide-prompt'.
(erc--prompt-hidden-p): New internal predicate function.
(erc--unhide-prompt): Defer to `erc--reveal-prompt' and set
`erc-prompt' text property to t.
(erc--hide-prompt): Defer to `erc--conceal-prompt' and set
`erc-prompt' text property to `hidden'.
* lisp/erc/erc-compat.el (erc-compat--29-browse-url-irc): Add FIXME
comment for likely insufficient test of function equality.
* lisp/erc/erc-fill.el (erc-fill-wrap-margin-width,
erc-fill-wrap-margin-side): New options to control side and initial
width of `fill-wrap' margin.
(erc-fill--wrap-beginning-of-line): Fix bug involving non-string
valued `display' props.
(erc-fill-wrap-mode, erc-fill-wrap-enable): Update doc string, persist
a few local vars, and conditionally set `erc-stamp--margin-left-p'.
(erc-fill-wrap-nudge): Update doc string and account for left-hand
stamps.
(erc-timestamp-offset): Add comment regarding conditional guard based
on function-valued option.
* lisp/erc/erc-stamp.el (erc-timestamp-use-align-to): Remove value
variant `margin', which was originally intended to be new in ERC 5.6.
This functionality was all but useless without the internal minor mode
`erc-stamp--display-margin-mode' active.
(erc-stamp-right-margin-width): Remove unused option new in 5.6.
(erc-stamp--display-margin-force): Remove unused function.
(erc-stamp--margin-width, erc-stamp--margin-left-p): New internal var.
(erc-stamp--margin-left-p, erc-stamp--init-margins-on-connect): New
functions for other modules that use `erc-stamp--display-margin-mode'.
(erc-stamp--adjust-right-margin, erc-stamp--adjust-margin): Rename
function to latter and accommodate left-hand stamps.
(erc-stamp--inherited-props): Relocate from lower down in file.
(erc-stamp--display-margin-mode): Update function name, and adjust
setup and teardown to accommodate left-handed stamps.  Don't add
advice around `erc-insert-timestamp-function'.
(erc-stamp--last-prompt, erc-stamp--display-prompt-in-left-margin):
New function and helper var to convert a normal inserted prompt so
that it appears in the left margin.
(erc-stamp--refresh-left-margin-prompt): Helper for other modules to
quickly refresh prompt outside of insert hooks.
(erc--reveal-prompt, erc--conceal-prompt): New implementations for
when `erc-stamp--display-margin-mode' is active.
(erc-insert-timestamp-left): Convert to defmethod and provide
implementation for `erc-stamp--display-margin-mode'.
(erc-insert-timestamp-right): Don't expect `erc-timestamp-align-to' to
ever be the symbol `margin'.  Move handling for that case to one
contingent on the internal minor mode `erc-stamp--display-margin-mode'
being active.
* lisp/erc/erc.el (erc--refresh-prompt-hook): New variable.
(erc--refresh-prompt): Fix bug in which user-defined prompt functions
failed to hide when quitting in server buffers.  Run new hook
`erc--refresh-prompt-hook'.
(erc-display-prompt): Add comment noting that the text property
`erc-prompt' now actually matters.  It's t while a session is running
and `hidden' when disconnected.
* test/lisp/erc/erc-fill-tests.el (erc-fill--left-hand-stamps): New
test.
* test/lisp/erc/erc-stamp-tests.el
(erc-timestamp-use-align-to--margin,
erc-stamp--display-margin-mode--right): Rename test to latter.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Add some assertions
for new possible value of `erc-prompt' text property.
* test/lisp/erc/resources/fill/snapshots/stamps-left-01.eld: New test
data file.  (Bug#60936)
---
 etc/ERC-NEWS                                  |   7 +-
 lisp/erc/erc-backend.el                       |  23 +-
 lisp/erc/erc-compat.el                        |   1 +
 lisp/erc/erc-fill.el                          |  76 +++++--
 lisp/erc/erc-stamp.el                         | 199 +++++++++++++-----
 lisp/erc/erc.el                               |  26 ++-
 test/lisp/erc/erc-fill-tests.el               |  37 ++++
 test/lisp/erc/erc-stamp-tests.el              |   2 +-
 test/lisp/erc/erc-tests.el                    |   6 +
 .../fill/snapshots/stamps-left-01.eld         |   1 +
 10 files changed, 281 insertions(+), 97 deletions(-)
 create mode 100644 test/lisp/erc/resources/fill/snapshots/stamps-left-01.eld

diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index cd0b8e5f823..379d5eb2ad0 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -102,11 +102,8 @@ side window.  Hit '<RET>' over a nick to spawn a "/QUERY" or a
 ** The option 'erc-timestamp-use-align-to' is more versatile.
 While this option has always offered to right-align stamps via the
 'display' text property, it's now more effective at doing so when set
-to a number indicating an offset from the right edge.  And when set to
-the symbol 'margin', it displays stamps in the right margin, although,
-at the moment, this is mostly intended for use by other modules, such
-as 'fill-wrap', described above.  For both these variants, users of
-the 'log' module may want to customize 'erc-log-filter-function' to
+to a number indicating an offset from the right edge.  Users of the
+'log' module may want to customize 'erc-log-filter-function' to
 'erc-stamp-prefix-log-filter' to avoid ragged right-hand stamps
 appearing in their saved logs.
 
diff --git a/lisp/erc/erc-backend.el b/lisp/erc/erc-backend.el
index 363509d17fa..eb3ec39fedd 100644
--- a/lisp/erc/erc-backend.el
+++ b/lisp/erc/erc-backend.el
@@ -1045,13 +1045,25 @@ erc-process-sentinel-1
       ;; unexpected disconnect
       (erc-process-sentinel-2 event buffer))))
 
+(cl-defmethod erc--reveal-prompt ()
+  (remove-text-properties erc-insert-marker erc-input-marker
+                          '(display nil)))
+
+(cl-defmethod erc--conceal-prompt ()
+  (add-text-properties erc-insert-marker (1- erc-input-marker)
+                       `(display ,erc-prompt-hidden)))
+
+(defun erc--prompt-hidden-p ()
+  (and (marker-position erc-insert-marker)
+       (eq (get-text-property erc-insert-marker 'erc-prompt) 'hidden)))
+
 (defun erc--unhide-prompt ()
   (remove-hook 'pre-command-hook #'erc--unhide-prompt-on-self-insert t)
   (when (and (marker-position erc-insert-marker)
              (marker-position erc-input-marker))
     (with-silent-modifications
-      (remove-text-properties erc-insert-marker erc-input-marker
-                              '(display nil)))))
+      (put-text-property erc-insert-marker (1- erc-input-marker) 'erc-prompt t)
+      (erc--reveal-prompt))))
 
 (defun erc--unhide-prompt-on-self-insert ()
   (when (and (eq this-command #'self-insert-command)
@@ -1059,6 +1071,8 @@ erc--unhide-prompt-on-self-insert
     (erc--unhide-prompt)))
 
 (defun erc--hide-prompt (proc)
+  "Hide prompt in all buffers of server.
+Change value of property `erc-prompt' from t to `hidden'."
   (erc-with-all-buffers-of-server proc nil
     (when (and erc-hide-prompt
                (or (eq erc-hide-prompt t)
@@ -1072,8 +1086,9 @@ erc--hide-prompt
                (marker-position erc-input-marker)
                (get-text-property erc-insert-marker 'erc-prompt))
       (with-silent-modifications
-        (add-text-properties erc-insert-marker (1- erc-input-marker)
-                             `(display ,erc-prompt-hidden)))
+        (put-text-property erc-insert-marker (1- erc-input-marker)
+                           'erc-prompt 'hidden)
+        (erc--conceal-prompt))
       (add-hook 'pre-command-hook #'erc--unhide-prompt-on-self-insert 91 t))))
 
 (defun erc-process-sentinel (cproc event)
diff --git a/lisp/erc/erc-compat.el b/lisp/erc/erc-compat.el
index f451aaee754..912a4bc576c 100644
--- a/lisp/erc/erc-compat.el
+++ b/lisp/erc/erc-compat.el
@@ -418,6 +418,7 @@ erc-compat--29-browse-url-irc
   (require 'url-irc)
   (let* ((url (url-generic-parse-url string))
          (url-irc-function
+          ;; FIXME this should probably use `symbol-function'.
           (if (function-equal url-irc-function 'url-irc-erc)
               (lambda (host port chan user pass)
                 (erc-handle-irc-url host port chan user pass (url-type url)))
diff --git a/lisp/erc/erc-fill.el b/lisp/erc/erc-fill.el
index a65c95f1d85..9f39f41133d 100644
--- a/lisp/erc/erc-fill.el
+++ b/lisp/erc/erc-fill.el
@@ -116,6 +116,25 @@ erc-fill-column
   "The column at which a filled paragraph is broken."
   :type 'integer)
 
+(defcustom erc-fill-wrap-margin-width nil
+  "Starting width in columns of dedicated stamp margin.
+When nil, ERC normally pretends its value is one column greater
+than the `string-width' of the formatted `erc-timestamp-format'.
+However, when `erc-fill-wrap-margin-side' is `left' or
+\"resolves\" to `left', ERC uses the width of the prompt if it's
+wider on MOTD's end, which really only matters when `erc-prompt'
+is a function."
+  :package-version '(ERC . "5.6") ; FIXME sync on release
+  :type '(choice (const nil) integer))
+
+(defcustom erc-fill-wrap-margin-side nil
+  "Margin side to use with `erc-fill-wrap-mode'.
+A value of nil means ERC should decide based on
+`erc-insert-timestamp-function', which obviously cannot work for
+user-defined functions."
+  :package-version '(ERC . "5.6") ; FIXME sync on release
+  :type '(choice (const nil) (const left) (const right)))
+
 (defcustom erc-fill-line-spacing nil
   "Extra space between messages on graphical displays.
 This may need adjusting depending on how your faces are
@@ -253,9 +272,9 @@ erc-fill--wrap-beginning-of-line
       (goto-char erc-input-marker)
     ;; Mimic what `move-beginning-of-line' does with invisible text.
     (when-let ((erc-fill-wrap-merge)
-               (empty (get-text-property (point) 'display))
-               ((string-empty-p empty)))
-      (goto-char (text-property-not-all (point) (pos-eol) 'display empty)))))
+               (prop (get-text-property (point) 'display))
+               ((or (equal prop "") (eq 'margin (car-safe (car-safe prop))))))
+      (goto-char (text-property-not-all (point) (pos-eol) 'display prop)))))
 
 (defun erc-fill--wrap-end-of-line (arg)
   "Defer to `move-end-of-line' or `end-of-visual-line'."
@@ -319,21 +338,33 @@ fill-wrap
   "Fill style leveraging `visual-line-mode'.
 This local module displays nicks overhanging leftward to a common
 offset, as determined by the option `erc-fill-static-center'.  It
-depends on the `fill' and `button' modules and assumes the option
-`erc-insert-timestamp-function' is `erc-insert-timestamp-right'
-or the default `erc-insert-timestamp-left-and-right', so that it
-can display right-hand stamps in the right margin.  A value of
-`erc-insert-timestamp-left' is unsupported.  To use it, either
-include `fill-wrap' in `erc-modules' or set `erc-fill-function'
-to `erc-fill-wrap' (recommended).  You can also manually invoke
-one of the minor-mode toggles if really necessary."
+depends on the `fill' and `button' modules and assumes users
+who've defined their own `erc-insert-timestamp-function' have
+also customized the option `erc-fill-wrap-margin-side' to an
+explicit side.  To use this module, either include `fill-wrap' in
+`erc-modules' or set `erc-fill-function' to
+`erc-fill-wrap' (recommended).  You can also manually invoke one
+of the minor-mode toggles if really necessary.
+
+When stamps appear in the right margin, which they do by default,
+users may find that ERC actually appends them to copy-as-killed
+messages without an intervening space.  This normally poses at
+most a minor nuisance, however users of the `log' module may
+prefer a workaround provided by `erc-stamp-prefix-log-filter',
+which strips trailing stamps from logged messages and instead
+prepends them to every line."
   ((erc-fill--wrap-ensure-dependencies)
-   ;; Restore or initialize local state variables.
    (erc--restore-initialize-priors erc-fill-wrap-mode
      erc-fill--wrap-visual-keys erc-fill-wrap-visual-keys
-     erc-fill--wrap-value erc-fill-static-center)
+     erc-fill--wrap-value erc-fill-static-center
+     erc-stamp--margin-width erc-fill-wrap-margin-width
+     left-margin-width 0
+     right-margin-width 0)
+   ;; Only give this a local binding if known for sure.
+   (pcase erc-fill-wrap-margin-side
+     ('right (setq erc-stamp--margin-left-p nil))
+     ('left (setq erc-stamp--margin-left-p t)))
    (setq erc-fill--function #'erc-fill-wrap)
-   ;; Internal integrations.
    (add-function :after (local 'erc-stamp--insert-date-function)
                  #'erc-fill--wrap-stamp-insert-prefixed-date)
    (when (or erc-stamp-mode (memq 'stamp erc-modules))
@@ -476,8 +507,8 @@ erc-fill-wrap-nudge
    \\`=' Increase indentation by one column
    \\`-' Decrease indentation by one column
    \\`0' Reset indentation to the default
-   \\`+' Shift right margin rightward (shrink) by one column
-   \\`_' Shift right margin leftward (grow) by one column
+   \\`+' Shift margin boundary rightward by one column
+   \\`_' Shift margin boundary leftward by one column
    \\`)' Reset the right margin to the default
 
 Note that misalignment may occur when messages contain
@@ -507,14 +538,16 @@ erc-fill-wrap-nudge
                          (cl-incf total (erc-fill--wrap-nudge a))
                          (recenter (round (* win-ratio (window-height))))))))
        (dolist (key '(?\) ?_ ?+))
-         (let ((a (pcase key
-                    (?\) 0)
-                    (?_ (- (abs arg)))
-                    (?+ (abs arg)))))
+         (let* ((leftp erc-stamp--margin-left-p)
+                (a (pcase key
+                     (?\) 0)
+                     (?_ (if leftp (abs arg) (- (abs arg))))
+                     (?+ (if leftp (- (abs arg)) (abs arg))))))
            (define-key map (vector (list key))
                        (lambda ()
                          (interactive)
-                         (erc-stamp--adjust-right-margin (- a))
+                         (erc-stamp--adjust-margin (- a) (zerop a))
+                         (when leftp (erc-stamp--refresh-left-margin-prompt))
                          (recenter (round (* win-ratio (window-height))))))))
        map)
      t
@@ -536,6 +569,7 @@ erc-timestamp-offset
   "Get length of timestamp if inserted left."
   (if (and (boundp 'erc-timestamp-format)
            erc-timestamp-format
+           ;; FIXME use a more robust test than symbol equivalence.
            (eq erc-insert-timestamp-function 'erc-insert-timestamp-left)
            (not erc-hide-timestamps))
       (length (format-time-string erc-timestamp-format))
diff --git a/lisp/erc/erc-stamp.el b/lisp/erc/erc-stamp.el
index 83ee4a200ed..727d334f13b 100644
--- a/lisp/erc/erc-stamp.el
+++ b/lisp/erc/erc-stamp.el
@@ -281,49 +281,67 @@ erc-timestamp-use-align-to
 set to `erc-insert-timestamp-right' or that option's default,
 `erc-insert-timestamp-left-and-right'.  If the value is a
 positive integer, alignment occurs that many columns from the
-right edge.  If the value is `margin', the stamp appears in the
-right margin when visible.
+right edge.
 
 Enabling this option produces a side effect in that stamps aren't
 indented in saved logs.  When its value is an integer, this
 option adds a space after the end of a message if the stamp
 doesn't already start with one.  And when its value is t, it adds
-a single space, unconditionally.  And while this option never
-adds a space when its value is `margin', ERC does offer a
-workaround in `erc-stamp-prefix-log-filter', which strips
-trailing stamps from messages and puts them before every line."
-  :type '(choice boolean integer (const margin))
+a single space, unconditionally."
+  :type '(choice boolean integer)
   :package-version '(ERC . "5.6")) ; FIXME sync on release
 
-(defcustom erc-stamp-right-margin-width nil
-  "Width in columns of the right margin.
-When this option is nil, pretend its value is one column greater
-than the `string-width' of the formatted `erc-timestamp-format'.
-This option only matters when `erc-timestamp-use-align-to' is set
-to `margin'."
-  :package-version '(ERC . "5.6") ; FIXME sync on release
-  :type '(choice (const nil) integer))
-
-(defun erc-stamp--display-margin-force (orig &rest r)
-  (let ((erc-timestamp-use-align-to 'margin))
-    (apply orig r)))
-
-(defun erc-stamp--adjust-right-margin (cols)
-  "Adjust right margin by COLS.
-When COLS is zero, reset width to `erc-stamp-right-margin-width'
-or one col more than the `string-width' of
-`erc-timestamp-format'."
-  (let ((width
-         (if (zerop cols)
-             (or erc-stamp-right-margin-width
-                 (1+ (string-width (or erc-timestamp-last-inserted-right
-                                       (erc-format-timestamp
-                                        (current-time)
-                                        erc-timestamp-format)))))
-           (+ right-margin-width cols))))
-    (setq right-margin-width width)
+(defvar-local erc-stamp--margin-width nil
+  "Width in columns of margin for `erc-stamp--display-margin-mode'.
+Only consulted when resetting or initializing margin.")
+
+(defvar-local erc-stamp--margin-left-p nil
+  "Whether `erc-stamp--display-margin-mode' uses the left margin.
+During initialization, the mode respects this variable's existing
+value if it already has a local binding.  Otherwise, modules can
+bind this to any value while enabling the mode.  If it's nil, ERC
+will check to see if `erc-insert-timestamp-function' is
+`erc-insert-timestamp-left', interpreting the latter as a non-nil
+value.  It'll then coerce any non-nil value to t.")
+
+(defun erc-stamp--margin-left-p (&optional value)
+  (and (or value
+           (function-equal (symbol-function (default-value
+                                             'erc-insert-timestamp-function))
+                           (symbol-function 'erc-insert-timestamp-left)))
+       t))
+
+(defun erc-stamp--init-margins-on-connect (&rest _)
+  (let ((existing (if erc-stamp--margin-left-p
+                      left-margin-width
+                    right-margin-width)))
+    (erc-stamp--adjust-margin existing 'resetp)))
+
+(defun erc-stamp--adjust-margin (cols &optional resetp)
+  "Adjust managed margin by increment COLS.
+With RESETP, set margin's width to COLS.  However, if COLS is
+zero, set the width to a non-nil `erc-stamp--margin-width'.
+Otherwise, go with the `string-width' of `erc-timestamp-format'.
+However, when `erc-stamp--margin-left-p' is non-nil and the
+prompt is wider, use its width instead."
+  (let* ((leftp erc-stamp--margin-left-p)
+         (width
+          (if resetp
+              (or (and (not (zerop cols)) cols)
+                  erc-stamp--margin-width
+                  (max (if leftp (string-width (erc-prompt)) 0)
+                       (1+ (string-width
+                            (or (if leftp
+                                    erc-timestamp-last-inserted
+                                  erc-timestamp-last-inserted-right)
+                                (erc-format-timestamp
+                                 (current-time) erc-timestamp-format))))))
+            (+ (if leftp left-margin-width right-margin-width) cols))))
+    (set (if leftp 'left-margin-width 'right-margin-width) width)
     (when (eq (current-buffer) (window-buffer))
-      (set-window-margins nil left-margin-width width))))
+      (set-window-margins nil
+                          (if leftp width left-margin-width)
+                          (if leftp right-margin-width width)))))
 
 ;;;###autoload
 (defun erc-stamp-prefix-log-filter (text)
@@ -348,39 +366,97 @@ erc-stamp-prefix-log-filter
         (zerop (forward-line))))
   "")
 
+(defvar erc-stamp--inherited-props '(line-prefix wrap-prefix))
+
 (declare-function erc--remove-text-properties "erc" (string))
 
-;; If people want to use this directly, we can convert it into
-;; a local module.
+;; If people want to use this directly, we can convert it into a local
+;; module.  Also, `erc-insert-timestamp-right' hard codes its display
+;; property to use `right-margin', and `erc-insert-timestamp-left'
+;; does the same for `left-margin'.  However, there's no reason a
+;; trailing stamp couldn't be displayed on the left and vice versa.
+;; Note: this adds advice that breaks `erc-timestamp-offset' because
+;; the thinking is there's no use case in which that function would be
+;; called while this mode is active.  See note below for more.
 (define-minor-mode erc-stamp--display-margin-mode
   "Internal minor mode for built-in modules integrating with `stamp'.
-It binds `erc-timestamp-use-align-to' to `margin' around calls to
-`erc-insert-timestamp-function' in the current buffer, and sets
-the right window margin to `erc-stamp-right-margin-width'.  It
-also arranges to remove most text properties when a user kills
-message text so that stamps will be visible when yanked."
+Manages chosen window margin and arranges to remove `display'
+text properties in killed text to reveal stamps."
   :interactive nil
   (if erc-stamp--display-margin-mode
       (progn
         (setq fringes-outside-margins t)
         (when (eq (current-buffer) (window-buffer))
           (set-window-buffer (selected-window) (current-buffer)))
-        (erc-stamp--adjust-right-margin 0)
+        (unless (local-variable-p 'erc-stamp--margin-left-p)
+          (setq erc-stamp--margin-left-p
+                (erc-stamp--margin-left-p erc-stamp--margin-left-p)))
+        (if (or erc-server-connected (not (functionp erc-prompt)))
+            (erc-stamp--init-margins-on-connect)
+          (add-hook 'erc-after-connect
+                    #'erc-stamp--init-margins-on-connect nil t))
         (add-function :filter-return (local 'filter-buffer-substring-function)
                       #'erc--remove-text-properties)
-        (add-function :around (local 'erc-insert-timestamp-function)
-                      #'erc-stamp--display-margin-force))
+        (when erc-stamp--margin-left-p
+          (add-hook 'erc--refresh-prompt-hook
+                    #'erc-stamp--display-prompt-in-left-margin nil t)))
     (remove-function (local 'filter-buffer-substring-function)
                      #'erc--remove-text-properties)
-    (remove-function (local 'erc-insert-timestamp-function)
-                     #'erc-stamp--display-margin-force)
-    (kill-local-variable 'right-margin-width)
+    (add-hook 'erc-after-connect #'erc-stamp--init-margins-on-connect t)
+    (remove-hook 'erc--refresh-prompt-hook
+                 #'erc-stamp--display-prompt-in-left-margin t)
+    (kill-local-variable (if erc-stamp--margin-left-p
+                             'left-margin-width
+                           'right-margin-width))
     (kill-local-variable 'fringes-outside-margins)
+    (kill-local-variable 'erc-stamp--margin-prompt-width)
+    (kill-local-variable 'erc-stamp--margin-left-p)
+    (kill-local-variable 'erc-stamp--margin-width)
     (when (eq (current-buffer) (window-buffer))
       (set-window-margins nil left-margin-width nil)
       (set-window-buffer (selected-window) (current-buffer)))))
 
-(defun erc-insert-timestamp-left (string)
+(defvar-local erc-stamp--last-prompt nil)
+
+(defun erc-stamp--display-prompt-in-left-margin ()
+  "Show prompt in the left margin with padding."
+  (when (or (not erc-stamp--last-prompt) (functionp erc-prompt)
+            (> (string-width erc-stamp--last-prompt) left-margin-width))
+    (let ((s (buffer-substring erc-insert-marker (1- erc-input-marker))))
+      ;; Prevent #("abc" n m (display ((...) #("abc" p q (display...))))
+      (remove-text-properties 0 (length s) '(display nil) s)
+      (when (and erc-stamp--last-prompt
+                 (>= (string-width erc-stamp--last-prompt) left-margin-width))
+        (let ((sm (truncate-string-to-width s (1- left-margin-width) 0 nil t)))
+          ;; This papers over a subtle off-by-1 bug here.
+          (unless (equal sm s)
+            (setq s (concat sm (substring s -1))))))
+      (setq erc-stamp--last-prompt (string-pad s left-margin-width nil t))))
+  (put-text-property erc-insert-marker (1- erc-input-marker)
+                     'display `((margin left-margin) ,erc-stamp--last-prompt))
+  erc-stamp--last-prompt)
+
+(defun erc-stamp--refresh-left-margin-prompt ()
+  "Forcefully-recompute display property of prompt in left margin."
+  (with-silent-modifications
+    (unless (functionp erc-prompt)
+      (setq erc-stamp--last-prompt nil))
+    (erc--refresh-prompt)))
+
+(cl-defmethod erc--reveal-prompt
+  (&context (erc-stamp--display-margin-mode (eql t))
+            (erc-stamp--margin-left-p (eql t)))
+  (put-text-property erc-insert-marker (1- erc-input-marker)
+                     'display `((margin left-margin) ,erc-stamp--last-prompt)))
+
+(cl-defmethod erc--conceal-prompt
+  (&context (erc-stamp--display-margin-mode (eql t))
+            (erc-stamp--margin-left-p (eql t)))
+  (let ((prompt (string-pad erc-prompt-hidden left-margin-width nil 'start)))
+    (put-text-property erc-insert-marker (1- erc-input-marker)
+                       'display `((margin left-margin) ,prompt))))
+
+(cl-defmethod erc-insert-timestamp-left (string)
   "Insert timestamps at the beginning of the line."
   (goto-char (point-min))
   (let* ((ignore-p (and erc-timestamp-only-if-changed-flag
@@ -392,6 +468,22 @@ erc-insert-timestamp-left
     (erc-put-text-property 0 len 'invisible erc-stamp--invisible-property s)
     (insert s)))
 
+(cl-defmethod erc-insert-timestamp-left
+  (string &context (erc-stamp--display-margin-mode (eql t)))
+  (unless (and erc-timestamp-only-if-changed-flag
+               (string-equal string erc-timestamp-last-inserted))
+    (goto-char (point-min))
+    (insert-before-markers-and-inherit
+     (setq erc-timestamp-last-inserted string))
+    (dolist (p erc-stamp--inherited-props)
+      (when-let ((v (get-text-property (point) p)))
+        (put-text-property (point-min) (point) p v)))
+    (erc-put-text-property (point-min) (point) 'invisible
+                           erc-stamp--invisible-property)
+    (put-text-property (point-min) (point) 'field 'erc-timestamp)
+    (put-text-property (point-min) (point)
+                       'display `((margin left-margin) ,string))))
+
 (defun erc-insert-aligned (string pos)
   "Insert STRING at the POSth column.
 
@@ -408,8 +500,6 @@ erc-insert-aligned
 ;; Silence byte-compiler
 (defvar erc-fill-column)
 
-(defvar erc-stamp--inherited-props '(line-prefix wrap-prefix))
-
 (defun erc-insert-timestamp-right (string)
   "Insert timestamp on the right side of the screen.
 STRING is the timestamp to insert.  This function is a possible
@@ -465,6 +555,9 @@ erc-insert-timestamp-right
       ;; For compatibility reasons, the `erc-timestamp' field includes
       ;; intervening white space unless a hard break is warranted.
       (pcase erc-timestamp-use-align-to
+        ((guard erc-stamp--display-margin-mode)
+         (put-text-property 0 (length string)
+                            'display `((margin right-margin) ,string) string))
         ((and 't (guard (< col pos)))
          (insert " ")
          (put-text-property from (point) 'display `(space :align-to ,pos)))
@@ -475,10 +568,6 @@ erc-insert-timestamp-right
          (let ((s (+ erc-timestamp-use-align-to (string-width string))))
            (put-text-property from (point) 'display
                               `(space :align-to (- right ,s)))))
-        ('margin
-         (put-text-property 0 (length string)
-                            'display `((margin right-margin) ,string)
-                            string))
         ((guard (>= col pos)) (newline) (indent-to pos) (setq from (point)))
         (_ (indent-to pos)))
       (insert string)
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 03c21059a92..c90f20cc9a4 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -2879,19 +2879,23 @@ erc--assert-input-bounds
           (cl-assert (< erc-insert-marker erc-input-marker))
           (cl-assert (= (field-end erc-insert-marker) erc-input-marker)))))
 
+(defvar erc--refresh-prompt-hook nil)
+
 (defun erc--refresh-prompt ()
   "Re-render ERC's prompt when the option `erc-prompt' is a function."
   (erc--assert-input-bounds)
-  (when (functionp erc-prompt)
-    (save-excursion
-      (goto-char erc-insert-marker)
-      (set-marker-insertion-type erc-insert-marker nil)
-      ;; Avoid `erc-prompt' (the named function), which appends a
-      ;; space, and `erc-display-prompt', which propertizes all but
-      ;; that space.
-      (insert-and-inherit (funcall erc-prompt))
-      (set-marker-insertion-type erc-insert-marker t)
-      (delete-region (point) (1- erc-input-marker)))))
+  (unless (erc--prompt-hidden-p)
+    (when (functionp erc-prompt)
+      (save-excursion
+        (goto-char erc-insert-marker)
+        (set-marker-insertion-type erc-insert-marker nil)
+        ;; Avoid `erc-prompt' (the named function), which appends a
+        ;; space, and `erc-display-prompt', which propertizes all but
+        ;; that space.
+        (insert-and-inherit (funcall erc-prompt))
+        (set-marker-insertion-type erc-insert-marker t)
+        (delete-region (point) (1- erc-input-marker))))
+    (run-hooks 'erc--refresh-prompt-hook)))
 
 (defun erc-display-line-1 (string buffer)
   "Display STRING in `erc-mode' BUFFER.
@@ -4804,7 +4808,7 @@ erc-display-prompt
         ;; shall remain part of the prompt.
         (setq prompt (propertize prompt
                                  'rear-nonsticky t
-                                 'erc-prompt t
+                                 'erc-prompt t ; t or `hidden'
                                  'field 'erc-prompt
                                  'front-sticky t
                                  'read-only t))
diff --git a/test/lisp/erc/erc-fill-tests.el b/test/lisp/erc/erc-fill-tests.el
index 99ec4a9635e..67622da9f3d 100644
--- a/test/lisp/erc/erc-fill-tests.el
+++ b/test/lisp/erc/erc-fill-tests.el
@@ -340,4 +340,41 @@ erc-fill-wrap-visual-keys--prompt
        (should (search-backward "ERC> " nil t))
        (execute-kbd-macro "\C-a")))))
 
+(ert-deftest erc-fill--left-hand-stamps ()
+  :tags '(:unstable)
+  (unless (>= emacs-major-version 29)
+    (ert-skip "Emacs version too low, missing `buffer-text-pixel-size'"))
+
+  (let ((erc-timestamp-only-if-changed-flag nil)
+        (erc-insert-timestamp-function #'erc-insert-timestamp-left))
+    (erc-fill-tests--wrap-populate
+     (lambda ()
+       (should (= 8 left-margin-width))
+       (pcase-let ((`((margin left-margin) ,displayed)
+                    (get-text-property erc-insert-marker 'display)))
+         (should (equal-including-properties
+                  displayed #("    ERC>" 4 8
+                              ( read-only t
+                                front-sticky t
+                                field erc-prompt
+                                erc-prompt t
+                                rear-nonsticky t
+                                font-lock-face erc-prompt-face)))))
+       (erc-fill-tests--compare "stamps-left-01")
+
+       (ert-info ("Shrink left margin by 1 col")
+         (erc-stamp--adjust-margin -1)
+         (with-silent-modifications (erc--refresh-prompt))
+         (should (= 7 left-margin-width))
+         (pcase-let ((`((margin left-margin) ,displayed)
+                      (get-text-property erc-insert-marker 'display)))
+           (should (equal-including-properties
+                    displayed #("   ERC>" 3 7
+                                ( read-only t
+                                  front-sticky t
+                                  field erc-prompt
+                                  erc-prompt t
+                                  rear-nonsticky t
+                                  font-lock-face erc-prompt-face))))))))))
+
 ;;; erc-fill-tests.el ends here
diff --git a/test/lisp/erc/erc-stamp-tests.el b/test/lisp/erc/erc-stamp-tests.el
index 6da7ed4503d..f6de087a09a 100644
--- a/test/lisp/erc/erc-stamp-tests.el
+++ b/test/lisp/erc/erc-stamp-tests.el
@@ -140,7 +140,7 @@ erc-timestamp-use-align-to--integer
        (should (eql ?\s (char-after (field-beginning (point)))))
        (should (eql ?\n (char-after (field-end (point)))))))))
 
-(ert-deftest erc-timestamp-use-align-to--margin ()
+(ert-deftest erc-stamp--display-margin-mode--right ()
   (erc-stamp-tests--insert-right
    (lambda ()
      (erc-stamp--display-margin-mode +1)
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index b5db5fe8764..fff3c4cb704 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -219,6 +219,7 @@ erc-hide-prompt
       (setq erc-hide-prompt '(server))
       (with-current-buffer "ServNet"
         (erc--hide-prompt erc-server-process)
+        (should (eq (get-text-property erc-insert-marker 'erc-prompt) 'hidden))
         (should (string= ">" (get-text-property erc-insert-marker 'display))))
 
       (with-current-buffer "#chan"
@@ -229,6 +230,7 @@ erc-hide-prompt
 
       (with-current-buffer "ServNet"
         (erc--unhide-prompt)
+        (should (eq (get-text-property erc-insert-marker 'erc-prompt) t))
         (should-not (get-text-property erc-insert-marker 'display))))
 
     (ert-info ("Value: channel")
@@ -242,7 +244,9 @@ erc-hide-prompt
 
       (with-current-buffer "#chan"
         (should (string= ">" (get-text-property erc-insert-marker 'display)))
+        (should (eq (get-text-property erc-insert-marker 'erc-prompt) 'hidden))
         (erc--unhide-prompt)
+        (should (eq (get-text-property erc-insert-marker 'erc-prompt) t))
         (should-not (get-text-property erc-insert-marker 'display))))
 
     (ert-info ("Value: query")
@@ -253,7 +257,9 @@ erc-hide-prompt
 
       (with-current-buffer "bob"
         (should (string= ">" (get-text-property erc-insert-marker 'display)))
+        (should (eq (get-text-property erc-insert-marker 'erc-prompt) 'hidden))
         (erc--unhide-prompt)
+        (should (eq (get-text-property erc-insert-marker 'erc-prompt) t))
         (should-not (get-text-property erc-insert-marker 'display)))
 
       (with-current-buffer "#chan"
diff --git a/test/lisp/erc/resources/fill/snapshots/stamps-left-01.eld b/test/lisp/erc/resources/fill/snapshots/stamps-left-01.eld
new file mode 100644
index 00000000000..f62b65cd170
--- /dev/null
+++ b/test/lisp/erc/resources/fill/snapshots/stamps-left-01.eld
@@ -0,0 +1 @@
+#("\n\n[00:00]*** This server is in debug mode and is logging all user I/O. If you do not wish for everything you send to be readable by the server owner(s), please disconnect.\n[00:00]<alice> bob: come, you are a tedious fool: to the purpose. What was done to Elbow's wife, that he hath cause to complain of? Come me to what was done to her.\n[00:00]<bob> alice: Either your unparagoned mistress is dead, or she's outprized by a trifle.\n" 2 9 (erc-timestamp 0 display (#4=(margin left-margin) #("[00:00]" 0 7 (invisible timestamp font-lock-face erc-timestamp-face))) field erc-timestamp wrap-prefix #1=(space :width 27) line-prefix #2=(space :width (- 27 (4)))) 9 171 (erc-timestamp 0 wrap-prefix #1# line-prefix #2#) 172 179 (erc-timestamp 0 display (#4# #("[00:00]" 0 7 (invisible timestamp font-lock-face erc-timestamp-face))) field erc-timestamp wrap-prefix #1# line-prefix #3=(space :width (- 27 (8)))) 179 180 (erc-timestamp 0 wrap-prefix #1# line-prefix #3# erc-command PRIVMSG) 180 185 (erc-timestamp 0 wrap-prefix #1# line-prefix #3# erc-command PRIVMSG) 185 187 (erc-timestamp 0 wrap-prefix #1# line-prefix #3# erc-command PRIVMSG) 187 190 (erc-timestamp 0 wrap-prefix #1# line-prefix #3# erc-command PRIVMSG) 190 303 (erc-timestamp 0 wrap-prefix #1# line-prefix #3# erc-command PRIVMSG) 303 304 (erc-timestamp 0 erc-command PRIVMSG) 304 336 (erc-timestamp 0 wrap-prefix #1# line-prefix #3# erc-command PRIVMSG) 337 344 (erc-timestamp 0 display (#4# #("[00:00]" 0 7 (invisible timestamp font-lock-face erc-timestamp-face))) field erc-timestamp wrap-prefix #1# line-prefix #5=(space :width (- 27 (6)))) 344 345 (erc-timestamp 0 wrap-prefix #1# line-prefix #5# erc-command PRIVMSG) 345 348 (erc-timestamp 0 wrap-prefix #1# line-prefix #5# erc-command PRIVMSG) 348 350 (erc-timestamp 0 wrap-prefix #1# line-prefix #5# erc-command PRIVMSG) 350 355 (erc-timestamp 0 wrap-prefix #1# line-prefix #5# erc-command PRIVMSG) 355 430 (erc-timestamp 0 wrap-prefix #1# line-prefix #5# erc-command PRIVMSG))
\ No newline at end of file
-- 
2.41.0