bug#52324: 29.0.50; "Padded" mode line lacks uniform border

[Protesilaos Stavrou <info@protesilaos.com>]

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Dear maintainers,

In an emacs -Q session with version 27 we could evaluate the following
to make the mode line look more spacious ("padded", so to speak):

    (setq x-underline-at-descent-line t)

    (set-face-attribute 'mode-line nil
                        :foreground "black"
                        :background "gray80"
                        :box '(:line-width 6 :color "gray80")
                        :overline "gray30"
                        :underline "gray30"
                        :distant-foreground 'unspecified)

Whereas with master, the same results in a box with interrupted lines.

I attach screenshots, for your convenience.

* The emacs-27-mode-line-padded.png shows how it used to work on Emacs
  27.

* The emacs-29-mode-line-padded.png exhibits the current stylistic
  inconsistency.

Furthermore, there exists an invisible button (?) to the left of the
"All" text.  The emacs-29-mode-line-padded-mouse-pointer-button.png
shows it and also captures the tooltip.  (Maybe these are related to the
same underlying change on how the mode line is produced, otherwise I can
file a separate bug report).

All the best,
Protesilaos

-- 
Protesilaos Stavrou
https://protesilaos.com

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