Fix navigation block placeholder overlap on Nav Menus page

[talldan]

Description

The placeholder on the navigation block is overlapping its panel on the nav menus page, which this PR fixes (see screenshots).

The placeholder uses a width: 100% in addition to padding, which with the default of box-sizing: content-box causes an overlap. Enabling box-sizing: border-box for elements in the nav menus page resolves the issue.

How has this been tested?

1. Visit the nav menus page (wp-admin/admin.php?page=gutenberg-navigation)
2. Create a new menu
3. Observe the placeholder.

Screenshots

Before

After

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[draganescu]

LGTM! 🛸

[github-actions]

Size Change: +50 B (0%)

Total Size: 835 kB

Filename	Size	Change
build/edit-navigation/style-rtl.css	857 B	+25 B (2%)
build/edit-navigation/style.css	856 B	+25 B (2%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.02 kB	0 B
build/annotations/index.js	3.62 kB	0 B
build/api-fetch/index.js	3.39 kB	0 B
build/autop/index.js	2.83 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.93 kB	0 B
build/block-directory/style-rtl.css	790 B	0 B
build/block-directory/style.css	791 B	0 B
build/block-editor/index.js	105 kB	0 B
build/block-editor/style-rtl.css	10.8 kB	0 B
build/block-editor/style.css	10.8 kB	0 B
build/block-library/editor-rtl.css	7.19 kB	0 B
build/block-library/editor.css	7.19 kB	0 B
build/block-library/index.js	118 kB	0 B
build/block-library/style-rtl.css	7.48 kB	0 B
build/block-library/style.css	7.48 kB	0 B
build/block-library/theme-rtl.css	683 B	0 B
build/block-library/theme.css	685 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	48.1 kB	0 B
build/components/index.js	182 kB	0 B
build/components/style-rtl.css	17.1 kB	0 B
build/components/style.css	17 kB	0 B
build/compose/index.js	6.67 kB	0 B
build/core-data/index.js	11.4 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.42 kB	0 B
build/date/index.js	5.47 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	569 B	0 B
build/dom/index.js	3.11 kB	0 B
build/edit-navigation/index.js	6.6 kB	0 B
build/edit-post/index.js	28.1 kB	0 B
build/edit-post/style-rtl.css	12.2 kB	0 B
build/edit-post/style.css	12.2 kB	0 B
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build/edit-widgets/style.css	4.69 kB	0 B
build/editor/editor-styles-rtl.css	425 B	0 B
build/editor/editor-styles.css	428 B	0 B
build/editor/index.js	44.3 kB	0 B
build/editor/style-rtl.css	5.07 kB	0 B
build/editor/style.css	5.08 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.64 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.51 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.13 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	5.29 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	14.8 kB	0 B
build/server-side-render/index.js	2.68 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.28 kB	0 B
build/url/index.js	4.02 kB	0 B
build/viewport/index.js	1.84 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

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