Nav Block: Fix toolbar overlap on navigation links

[talldan]

Description

Fixes #20852

This is a somewhat brute-force attempt to resolve this issue by removed a z-index rule.

In testing, I couldn't see that removing this causes any issues. My suggestion would be that we try to live without z-index rules for as long as possible, unless we absolutely need it.

It might be that some themes require z-index rules, but then those should ideally be applied by the theme, because there's no way the core styles can get it right for everyone.

How has this been tested?

1. Add a nav block
2. Add a nav link and add multiple sub-links
3. Observe that when various links are selected, the toolbar is never overlapped.

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.

[github-actions]

Size Change: -12 B (0%)

Total Size: 856 kB

Filename	Size	Change
build/block-library/style-rtl.css	7.41 kB	-6 B (0%)
build/block-library/style.css	7.42 kB	-6 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	998 B	0 B
build/annotations/index.js	3.43 kB	0 B
build/api-fetch/index.js	3.39 kB	0 B
build/autop/index.js	2.58 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.02 kB	0 B
build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	760 B	0 B
build/block-editor/index.js	100 kB	0 B
build/block-editor/style-rtl.css	10.9 kB	0 B
build/block-editor/style.css	10.9 kB	0 B
build/block-library/editor-rtl.css	7.21 kB	0 B
build/block-library/editor.css	7.21 kB	0 B
build/block-library/index.js	110 kB	0 B
build/block-library/theme-rtl.css	669 B	0 B
build/block-library/theme.css	671 B	0 B
build/block-serialization-default-parser/index.js	1.65 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	57.5 kB	0 B
build/components/index.js	191 kB	0 B
build/components/style-rtl.css	15.8 kB	0 B
build/components/style.css	15.7 kB	0 B
build/compose/index.js	6.21 kB	0 B
build/core-data/index.js	10.6 kB	0 B
build/data-controls/index.js	1.04 kB	0 B
build/data/index.js	8.2 kB	0 B
build/date/index.js	5.37 kB	0 B
build/deprecated/index.js	771 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.06 kB	0 B
build/edit-post/index.js	91.2 kB	0 B
build/edit-post/style-rtl.css	8.47 kB	0 B
build/edit-post/style.css	8.46 kB	0 B
build/edit-site/index.js	5.56 kB	0 B
build/edit-site/style-rtl.css	2.62 kB	0 B
build/edit-site/style.css	2.62 kB	0 B
build/edit-widgets/index.js	4.43 kB	0 B
build/edit-widgets/style-rtl.css	2.58 kB	0 B
build/edit-widgets/style.css	2.58 kB	0 B
build/editor/editor-styles-rtl.css	381 B	0 B
build/editor/editor-styles.css	382 B	0 B
build/editor/index.js	43.8 kB	0 B
build/editor/style-rtl.css	3.97 kB	0 B
build/editor/style.css	3.96 kB	0 B
build/element/index.js	4.44 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	6.95 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	1.93 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.49 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.3 kB	0 B
build/keycodes/index.js	1.69 kB	0 B
build/list-reusable-blocks/index.js	2.99 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	4.83 kB	0 B
build/notices/index.js	1.58 kB	0 B
build/nux/index.js	3.01 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.54 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	780 B	0 B
build/redux-routine/index.js	2.83 kB	0 B
build/rich-text/index.js	14.4 kB	0 B
build/server-side-render/index.js	2.55 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.01 kB	0 B
build/viewport/index.js	1.61 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.18 kB	0 B

_{compressed-size-action}

[draganescu]

LGTM and I also agree that z-index should be last resort in the editor, there already are impossible riddles (see inserter and block UI).