[Fix #3219] Speed up printing output in the REPL buffer

I've opted to disable by default forcing the display of output when the REPL prompt is at the first line of the of the REPL window. This behavior is desirable, but very slow and rarely needed. It can be re-enabled by setting `cider-repl-display-output-before-window-boundaries` to `t`.

Author: bbatsov

CHANGELOG.md

@@ -15,6 +15,7 @@
 - Allow using `npx nbb` as `cider-nbb-command`.
 - [#3281](https://github.com/clojure-emacs/cider/pull/3281): Replace newline chars with actual newlines in `*cider-test-report*` buffer, for prettier error messages.
 - Bump the injected `cider-nrepl` to 0.30.
+- [#3219](https://github.com/clojure-emacs/cider/issues/3219): Disable by default forcing the display of output when the REPL prompt is at the first line of the of the REPL window. This behavior is desirable, but very slow and rarely needed. It can be re-enabled by setting `cider-repl-display-output-before-window-boundaries` to `t`.
 
 ## 1.6.0 (2022-12-21)
 

cider-repl.el

@@ -168,6 +168,19 @@ Will be evaluated with bindings for set!-able vars in place."
   :type 'boolean
   :package-version '(cider . "0.11.0"))
 
+;; See https://github.com/clojure-emacs/cider/issues/3219 for more details
+(defcustom cider-repl-display-output-before-window-boundaries nil
+  "Controls whether to display output emitted before the REPL window boundaries.
+
+If the prompt is on the first line of the window, then scroll the window
+down by a single line to make the emitted output visible.
+
+That behavior is desirable, but rarely needed and it slows down printing
+output a lot (e.g. 10x) that's why it's disable by default starting with
+CIDER 1.7."
+  :type 'boolean
+  :package-version '(cider . "1.7.0"))
+
 
 ;;;; REPL buffer local variables
 (defvar-local cider-repl-input-start-mark nil)
@@ -778,14 +791,16 @@ Before inserting, run `cider-repl-preoutput-hook' on STRING."
                  (not (bolp)))
         (insert-before-markers "\n")
         (set-marker cider-repl-output-end (1- (point))))))
-  (when-let* ((window (get-buffer-window buffer t)))
-    ;; If the prompt is on the first line of the window, then scroll the window
-    ;; down by a single line to make the emitted output visible.
-    (when (and (pos-visible-in-window-p cider-repl-prompt-start-mark window)
-               (< 1 cider-repl-prompt-start-mark)
-               (not (pos-visible-in-window-p (1- cider-repl-prompt-start-mark) window)))
-      (with-selected-window window
-        (scroll-down 1)))))
+  (when cider-repl-display-output-before-window-boundaries
+    ;; FIXME: The code below is super slow, that's why it's disabled by default.
+    (when-let* ((window (get-buffer-window buffer t)))
+      ;; If the prompt is on the first line of the window, then scroll the window
+      ;; down by a single line to make the emitted output visible.
+      (when (and (pos-visible-in-window-p cider-repl-prompt-start-mark window)
+                 (< 1 cider-repl-prompt-start-mark)
+                 (not (pos-visible-in-window-p (1- cider-repl-prompt-start-mark) window)))
+        (with-selected-window window
+          (scroll-down 1))))))
 
 (defun cider-repl--emit-interactive-output (string face)
   "Emit STRING as interactive output using FACE."