Small UX improvements for the nREPL messages buffer

CHANGELOG.md

@@ -12,6 +12,7 @@
 - New `cider-repl-history-doctor` command: walks `cider-repl-input-history` looking for entries whose parens don't balance under Clojure syntax, shows each in a side buffer, and asks whether to delete it.  When done, rewrites `cider-repl-history-file` if one is configured.  Useful for cleaning up history after a typo got committed that breaks `cider-repl-history` rendering (see [#3915](https://github.com/clojure-emacs/cider/issues/3915)).
 - Recognize [let-go](https://github.com/nooga/let-go) (a Clojure dialect implemented in Go) as a known nREPL runtime.  `cider-runtime` returns `let-go` for these connections and the connection info line shows the runtime version, e.g. `CLJ project@localhost:2137 (let-go 1.0)`.
 - Decouple the nREPL transport layer from CIDER's UI layer (closes [#1099](https://github.com/clojure-emacs/cider/issues/1099)). `nrepl-make-eval-handler` is now CIDER-agnostic: it no longer references `nrepl-namespace-handler-function`, `nrepl-err-handler-function`, `nrepl-need-input-handler-function`, or any hardcoded UI strings. New `:on-ns` and `:on-status` keyword slots let any consumer wire up their own namespace tracking and status handling. The editor-level `cider-make-eval-handler` wraps it with CIDER's UI behavior (ns tracking, default error handler, need-input prompt, "Evaluation interrupted." / "Namespace not found." messages); in-tree callers all use it.
+- New nREPL message log commands: `nrepl-show-messages` pops up the messages buffer (prompting when there are multiple connections), and `c` in `nrepl-messages-mode` runs the new `nrepl-clear-messages` to wipe the buffer. Auto-follow on new entries now leaves windows alone if you've scrolled back to read history; only windows already at end-of-buffer get scrolled to the latest message.
 
 ### Bugs fixed
 

lisp/nrepl-client.el

@@ -1233,6 +1233,7 @@ operations.")
   (let ((map (make-sparse-keymap)))
     (define-key map (kbd "n")   #'next-line)
     (define-key map (kbd "p")   #'previous-line)
+    (define-key map (kbd "c")   #'nrepl-clear-messages)
     (define-key map (kbd "TAB") #'forward-button)
     (define-key map (kbd "RET") #'nrepl-log-expand-button)
     (define-key map (kbd "e")   #'nrepl-log-expand-button)
@@ -1267,19 +1268,26 @@ described by `nrepl-message-buffer-name-template'."
     (setq msg (cons (car msg)
                     (nrepl-plist-put (cdr msg) "time-stamp"
                                      (format-time-string "%Y-%m-%0d %H:%M:%S.%N"))))
-    (with-current-buffer (nrepl-messages-buffer (current-buffer))
-      (setq buffer-read-only nil)
-      (when (> (buffer-size) nrepl-message-buffer-max-size)
-        (goto-char (/ (buffer-size) nrepl-message-buffer-reduce-denominator))
-        (re-search-forward "^(" nil t)
-        (delete-region (point-min) (- (point) 1)))
-      (goto-char (point-max))
-      (nrepl-log-pp-object (nrepl-decorate-msg msg type)
-                           (nrepl-log--message-color (nrepl-plist-get (cdr msg) "id"))
-                           t)
-      (when-let* ((win (get-buffer-window)))
-        (set-window-point win (point-max)))
-      (setq buffer-read-only t))))
+    (let ((log-buffer (nrepl-messages-buffer (current-buffer))))
+      (with-current-buffer log-buffer
+        (setq buffer-read-only nil)
+        ;; Snapshot which windows were already at end-of-buffer before we
+        ;; insert, so we only auto-follow those.  Windows the user has
+        ;; scrolled back to read history are left where they are.
+        (let ((following (seq-filter
+                          (lambda (w) (= (window-point w) (point-max)))
+                          (get-buffer-window-list log-buffer nil t))))
+          (when (> (buffer-size) nrepl-message-buffer-max-size)
+            (goto-char (/ (buffer-size) nrepl-message-buffer-reduce-denominator))
+            (re-search-forward "^(" nil t)
+            (delete-region (point-min) (- (point) 1)))
+          (goto-char (point-max))
+          (nrepl-log-pp-object (nrepl-decorate-msg msg type)
+                               (nrepl-log--message-color (nrepl-plist-get (cdr msg) "id"))
+                               t)
+          (dolist (w following)
+            (set-window-point w (point-max))))
+        (setq buffer-read-only t)))))
 
 (defun nrepl-toggle-message-logging ()
   "Toggle the value of `nrepl-log-messages' between nil and t.
@@ -1291,6 +1299,33 @@ This in effect enables or disables the logging of nREPL messages."
       (message "nREPL message logging enabled")
     (message "nREPL message logging disabled")))
 
+(defun nrepl-clear-messages ()
+  "Wipe the current nREPL messages buffer."
+  (interactive)
+  (unless (derived-mode-p 'nrepl-messages-mode)
+    (user-error "Not in an `nrepl-messages-mode' buffer"))
+  (let ((inhibit-read-only t))
+    (erase-buffer)))
+
+(defun nrepl-show-messages ()
+  "Pop up an nREPL messages buffer.
+With more than one active connection, prompt for which one to show.
+If `nrepl-log-messages' is nil no buffer exists yet, so signal an
+error pointing at `nrepl-toggle-message-logging'."
+  (interactive)
+  (let ((buffers (seq-filter (lambda (b)
+                               (with-current-buffer b
+                                 (derived-mode-p 'nrepl-messages-mode)))
+                             (buffer-list))))
+    (pcase (length buffers)
+      (0 (user-error "No nREPL messages buffer exists; enable logging with `nrepl-toggle-message-logging'"))
+      (1 (pop-to-buffer (car buffers)))
+      (_ (pop-to-buffer
+          (get-buffer
+           (completing-read "nREPL messages buffer: "
+                            (mapcar #'buffer-name buffers)
+                            nil t)))))))
+
 (defcustom nrepl-message-colors
   '("red" "brown" "coral" "orange" "green" "deep sky blue" "blue" "dark violet")
   "Colors used in the messages buffer."