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

[Eli Zaretskii <eliz@gnu.org>]

> 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.