bug#70594: 30.0.50; Error when tabbing with only one active widget

bug#70594: 30.0.50; Error when tabbing with only one active widget

[Stephen Berman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

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Commit 91333dacfa1, which adds the feature of optionally skipping over
inactive widgets when tabbing (bug#70413), fails to handle buffers that
contain only one active widget; in that case, pressing TAB or S-TAB
raises the error "No buttons or fields found".  To reproduce:

0. emacs -Q
1. Load the attached file widget-test.el and then type `M-x
   my-widget-test'.
2. In the buffer "*My Widget Test*" repeatedly press TAB and S-TAB and
   observe that point moves successively between the active radio
   buttons labelled "One" and "Two" and the active push button "Submit",
   but skips the inactive push button "Reset", since the user option
   widget-skip-inactive is enabled.
3. Now press (via RET or mouse-click) the "Submit" button, which makes
   the radio buttons and the "Submit" button inactive and the "Reset"
   button active.
4. Pressing TAB once moves point to the "Reset" button, then pressing
   TAB again moves point to BOB and raises the error "No buttons or
   fields found".  Likewise, pressing S-TAB once moves point to "Reset",
   then pressing S-TAB again moves to EOB and raises the same error.

The attached patched fixes this bug.  In addition, when only one widget
is active and point is on it, since pressing TAB or S-TAB does not move
point, the informational message "Only one tabable widget" is displayed.

The original code in widget-move contained separate loops to handle
forward and backward movement and this involved some code duplication.
The changes to this code to fix the bug would have required further
duplication, so I took the opportunity to parameterize the
directionality, allowing handling both directions in a single loop.
This shortens the code and IMO makes the similarities and differences in
the handling easier to see.


Configured using:
 'configure 'CFLAGS=-Og -g3' PKG_CONFIG_PATH=/opt/qt5/lib/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG
RSVG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER
WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB


[-- Attachment #2: widget-test.el --]
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[-- Attachment #3: widget-move patch --]
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diff --git a/lisp/wid-edit.el b/lisp/wid-edit.el
index dc481d4d0a5..407309c3429 100644
--- a/lisp/wid-edit.el
+++ b/lisp/wid-edit.el
@@ -1263,42 +1263,40 @@ widget-move
 ARG may be negative to move backward.
 When the second optional argument is non-nil,
 nothing is shown in the echo area."
-  (let ((wrapped 0)
-	(number arg)
-	(old (widget-tabable-at)))
-    ;; Forward.
-    (while (> arg 0)
-      (cond ((eobp)
-	     (goto-char (point-min))
-	     (setq wrapped (1+ wrapped)))
-	    (widget-use-overlay-change
-	     (goto-char (next-overlay-change (point))))
-	    (t
-	     (forward-char 1)))
-      (and (= wrapped 2)
-	   (eq arg number)
-	   (error "No buttons or fields found"))
-      (let ((new (widget-tabable-at)))
-	(when new
-	  (unless (eq new old)
-	    (setq arg (1- arg))
-	    (setq old new)))))
-    ;; Backward.
-    (while (< arg 0)
-      (cond ((bobp)
-	     (goto-char (point-max))
-	     (setq wrapped (1+ wrapped)))
-	    (widget-use-overlay-change
-	     (goto-char (previous-overlay-change (point))))
-	    (t
-	     (backward-char 1)))
-      (and (= wrapped 2)
-	   (eq arg number)
-	   (error "No buttons or fields found"))
-      (let ((new (widget-tabable-at)))
-	(when new
-	  (unless (eq new old)
-	    (setq arg (1+ arg))))))
+  (let* ((wrapped 0)
+	 (number arg)
+         (fwd (> arg 0))                ; widget-forward is caller.
+         (bwd (< arg 0))                ; widget-backward is caller.
+	 (old (widget-tabable-at))
+         (tabable (if old 1 0))
+         pos)
+    (catch 'one
+      (while (> (abs arg) 0)
+        (cond ((or (and fwd (eobp)) (and bwd (bobp)))
+	       (goto-char (cond (fwd (point-min))
+                                (bwd (point-max))))
+	       (setq wrapped (1+ wrapped)))
+	      (widget-use-overlay-change
+	       (goto-char (cond (fwd (next-overlay-change (point)))
+                                (bwd (previous-overlay-change (point))))))
+	      (t
+	       (cond (fwd (forward-char 1))
+                     (bwd (backward-char 1)))))
+        (and (= wrapped 2)
+	     (eq arg number)
+             (if (= tabable 1)
+                 (progn
+                   (goto-char pos)
+                   (throw 'one (message "Only one tabable widget")))
+	       (error "No buttons or fields found")))
+        (let ((new (widget-tabable-at)))
+	  (when new
+	    (if (eq new old)
+                (setq pos (point))
+              (cl-incf tabable)
+	      (setq arg (cond (fwd (1- arg))
+                              (bwd (1+ arg))))
+	      (setq old new))))))
     (let ((new (widget-tabable-at)))
       (while (and (eq (widget-tabable-at) new) (not (bobp)))
 	(backward-char)))

bug#70594: 30.0.50; Error when tabbing with only one active widget

[Eli Zaretskii]

> Date: Fri, 26 Apr 2024 14:43:42 +0200
> From:  Stephen Berman via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Commit 91333dacfa1, which adds the feature of optionally skipping over
> inactive widgets when tabbing (bug#70413), fails to handle buffers that
> contain only one active widget; in that case, pressing TAB or S-TAB
> raises the error "No buttons or fields found".  To reproduce:
> 
> 0. emacs -Q
> 1. Load the attached file widget-test.el and then type `M-x
>    my-widget-test'.
> 2. In the buffer "*My Widget Test*" repeatedly press TAB and S-TAB and
>    observe that point moves successively between the active radio
>    buttons labelled "One" and "Two" and the active push button "Submit",
>    but skips the inactive push button "Reset", since the user option
>    widget-skip-inactive is enabled.
> 3. Now press (via RET or mouse-click) the "Submit" button, which makes
>    the radio buttons and the "Submit" button inactive and the "Reset"
>    button active.
> 4. Pressing TAB once moves point to the "Reset" button, then pressing
>    TAB again moves point to BOB and raises the error "No buttons or
>    fields found".  Likewise, pressing S-TAB once moves point to "Reset",
>    then pressing S-TAB again moves to EOB and raises the same error.
> 
> The attached patched fixes this bug.  In addition, when only one widget
> is active and point is on it, since pressing TAB or S-TAB does not move
> point, the informational message "Only one tabable widget" is displayed.
> 
> The original code in widget-move contained separate loops to handle
> forward and backward movement and this involved some code duplication.
> The changes to this code to fix the bug would have required further
> duplication, so I took the opportunity to parameterize the
> directionality, allowing handling both directions in a single loop.
> This shortens the code and IMO makes the similarities and differences in
> the handling easier to see.

Adding Mauro to the discussion.