Fix Gnus summary widget navigation across frames

* lisp/gnus/gnus-sum.el (gnus-summary-widget-forward)
(gnus-summary-widget-backward): Signal more informative error if
article window is not found.  Consider other frames displaying
article buffer, and raise its frame before navigating its
widgets. (bug#35565)
* lisp/gnus/gnus-win.el (gnus-get-buffer-window): Simplify and add
docstring.

2 files changed, 22 insertions, 15 deletions

diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index b8aa302f11a..ac222acfd2d 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -9423,8 +9423,11 @@ With optional ARG, move across that many fields."
   (interactive "p")
   (gnus-summary-select-article)
   (gnus-configure-windows 'article)
-  (select-window (gnus-get-buffer-window gnus-article-buffer))
-  (widget-forward arg))
+  (let ((win (or (gnus-get-buffer-window gnus-article-buffer t)
+                 (error "No article window found"))))
+    (select-window win)
+    (select-frame-set-input-focus (window-frame win))
+    (widget-forward arg)))
 
 (defun gnus-summary-widget-backward (arg)
   "Move point to the previous field or button in the article.
@@ -9432,10 +9435,13 @@ With optional ARG, move across that many fields."
   (interactive "p")
   (gnus-summary-select-article)
   (gnus-configure-windows 'article)
-  (select-window (gnus-get-buffer-window gnus-article-buffer))
-  (unless (widget-at (point))
-    (goto-char (point-max)))
-  (widget-backward arg))
+  (let ((win (or (gnus-get-buffer-window gnus-article-buffer t)
+                 (error "No article window found"))))
+    (select-window win)
+    (select-frame-set-input-focus (window-frame win))
+    (unless (widget-at (point))
+      (goto-char (point-max)))
+    (widget-backward arg)))
 
 (defun gnus-summary-isearch-article (&optional regexp-p)
   "Do incremental search forward on the current article.
diff --git a/lisp/gnus/gnus-win.el b/lisp/gnus/gnus-win.el
index 5f7154c5456..a992fe731cb 100644
--- a/lisp/gnus/gnus-win.el
+++ b/lisp/gnus/gnus-win.el
@@ -28,6 +28,7 @@
 
 (require 'gnus)
 (require 'gnus-util)
+(require 'seq)
 
 (defgroup gnus-windows nil
   "Window configuration."
@@ -509,15 +510,15 @@ should have point."
               (delq lowest-buf bufs)))))
 
 (defun gnus-get-buffer-window (buffer &optional frame)
-  (cond ((and (null gnus-use-frames-on-any-display)
-              (memq frame '(t 0 visible)))
-         (car
-          (let ((frames (frames-on-display-list)))
-            (seq-remove (lambda (win) (not (memq (window-frame win)
-                                                     frames)))
-                            (get-buffer-window-list buffer nil frame)))))
-        (t
-         (get-buffer-window buffer frame))))
+  "Return a window currently displaying BUFFER, or nil if none.
+Like `get-buffer-window', but respecting
+`gnus-use-frames-on-any-display'."
+  (if (and (not gnus-use-frames-on-any-display)
+           (memq frame '(t 0 visible)))
+      (let ((frames (frames-on-display-list)))
+        (seq-find (lambda (win) (memq (window-frame win) frames))
+                  (get-buffer-window-list buffer nil frame)))
+    (get-buffer-window buffer frame)))
 
 (provide 'gnus-win)