Glitchy rendering when scrolling large output

[contrapunctus-1]

Minimal reproducible example -

(in-package :common-lisp-user)
(defpackage "APP"
  (:use :clim :clim-lisp)
  (:export "APP-MAIN"))
(in-package :app)

(define-application-frame superapp ()
  ((dir :initform (error "no dir provided")
        :initarg :dir
        :accessor dir))
  (:pointer-documentation t)
  (:panes
   (app :application
        :height (graft-height (find-graft))
        :width (graft-width (find-graft))
        :incremental-redisplay t
        :display-function 'display-app)
   (int :interactor :height 200 :width 600))
  (:layouts
   (default (vertically () app int))))

(defun display-app (frame pane)
  (loop for file in (uiop:directory-files (dir frame))
        do (with-open-file (stream file)
             (loop for char = (ignore-errors (read-char stream))
                   while char
                   do (format pane "~a" char)))))

(defun app-main (dir)
  (run-frame-top-level (make-application-frame 'superapp :dir dir)))

;; Run with a directory containing many text files. For example -
;; (app-main #P"/media/data/contrapunctus/git/cl/McCLIM/Core/clim-core/presentations/")

Observations -

1. Scrolling by using the mousewheel or dragging the scroll bar presents garbled output, as if old output was not cleared.
2. Switching to another application and back corrects CLIM's display.
3. The presence or absence of :incremental-redisplay t has no effect on this problem.

(clim-lisp:software-version) ;; ⇒ "5.17.0-1-amd64"

[john-a-carroll]

Seems very similar to #1188