Fix problem with 'scroll-bar-adjust-thumb-portion' nil (Bug#32002)

* lisp/scroll-bar.el (scroll-bar-drag-1): Do not scroll window
when its buffer is fully visible and
'scroll-bar-adjust-thumb-portion' is nil (Bug#32002).

1 files changed, 16 insertions, 8 deletions

diff --git a/lisp/scroll-bar.el b/lisp/scroll-bar.el
index 4d1ad03fa5f..7efbfc77742 100644
--- a/lisp/scroll-bar.el
+++ b/lisp/scroll-bar.el
@@ -254,14 +254,22 @@ EVENT should be a scroll bar click or drag event."
   (let* ((start-position (event-start event))
          (window (nth 0 start-position))
          (portion-whole (nth 2 start-position)))
-    (save-excursion
-      (with-current-buffer (window-buffer window)
-        ;; Calculate position relative to the accessible part of the buffer.
-        (goto-char (+ (point-min)
-                      (scroll-bar-scale portion-whole
-                                        (- (point-max) (point-min)))))
-        (vertical-motion 0 window)
-        (set-window-start window (point))))))
+    ;; With 'scroll-bar-adjust-thumb-portion' nil and 'portion-whole'
+    ;; indicating that the buffer is fully visible, do not scroll the
+    ;; window since that might make it impossible to scroll it back
+    ;; with GTK's thumb (Bug#32002).
+    (when (or scroll-bar-adjust-thumb-portion
+              (not (numberp (car portion-whole)))
+              (not (numberp (cdr portion-whole)))
+              (/= (car portion-whole) (cdr portion-whole)))
+      (save-excursion
+        (with-current-buffer (window-buffer window)
+          ;; Calculate position relative to the accessible part of the buffer.
+          (goto-char (+ (point-min)
+                        (scroll-bar-scale portion-whole
+                                          (- (point-max) (point-min)))))
+          (vertical-motion 0 window)
+          (set-window-start window (point)))))))
 
 (defun scroll-bar-drag (event)
   "Scroll the window by dragging the scroll bar slider.