bug#39143: 26.3; `widget-complete' in `customize-option'

[Drew Adams <drew.adams@oracle.com>]

emacs -Q

`M-x customize-option default-frame-alist'

Click `INS'.

Enter `cursor-color' for field `Parameter: '.

Put cursor at beginning of field `Value: ', and delete the default
value, `nil'.

Use `M-TAB' (or `ESC TAB').

You get this error/message: "Not in an editable field".

Two parts to this report.  The first is a bug; the second is an
enhancement request for handling of option `default-frame-alist' and
other frame-alist options.

1. That message is incorrect.  The cursor is definitely in an editable
field.  (Same incorrect message if you first type `Blu', then `M-TAB'.)

2. Emacs should provide reasonable completion for both fields,
`Parameter' and `Value', for `default-frame-alist'.

2a. Completion can be lax for `Parameter', providing candidates that are
predefined parameters, but letting you enter other values too.

2b. Completion can perhaps be strict for `Value' for some known
parameters, but it would probably be better if it were lax in all cases,
unless there is a strict requirement about the possible values in some
case.


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29 built on CIRROCUMULUS
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor 'Microsoft Corp.', version 10.0.17763