[Widgets] Fix toolbar not fixed to top and not horizontally scrollable.

[kevin940726]

Description

Close #25201 (comment).

How has this been tested?

1. Go to the customizer
2. Click Widget blocks.
3. Scroll it vertically, the toolbar should be fixed to top.
4. Click on any widget, scroll the toolbar horizontally, it should be scrollable.

Screenshots

Types of changes

Bug fix

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: +170 B (0%)

Total Size: 1.17 MB

Filename	Size	Change
build/edit-widgets/index.js	17.6 kB	+7 B (0%)
build/edit-widgets/style-rtl.css	2.88 kB	+82 B (2%)
build/edit-widgets/style.css	2.88 kB	+81 B (2%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.52 kB	0 B
build/api-fetch/index.js	3.35 kB	0 B
build/autop/index.js	2.72 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	8.53 kB	0 B
build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
build/block-editor/index.js	128 kB	0 B
build/block-editor/style-rtl.css	11.1 kB	0 B
build/block-editor/style.css	11.1 kB	0 B
build/block-library/editor-rtl.css	8.58 kB	0 B
build/block-library/editor.css	8.58 kB	0 B
build/block-library/index.js	135 kB	0 B
build/block-library/style-rtl.css	7.61 kB	0 B
build/block-library/style.css	7.6 kB	0 B
build/block-library/theme-rtl.css	741 B	0 B
build/block-library/theme.css	741 B	0 B
build/block-serialization-default-parser/index.js	1.78 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	47.5 kB	0 B
build/components/index.js	167 kB	0 B
build/components/style-rtl.css	15.5 kB	0 B
build/components/style.css	15.5 kB	0 B
build/compose/index.js	9.42 kB	0 B
build/core-data/index.js	12 kB	0 B
build/data-controls/index.js	1.27 kB	0 B
build/data/index.js	8.43 kB	0 B
build/date/index.js	31.9 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	4.42 kB	0 B
build/edit-navigation/index.js	10.7 kB	0 B
build/edit-navigation/style-rtl.css	868 B	0 B
build/edit-navigation/style.css	871 B	0 B
build/edit-post/index.js	306 kB	0 B
build/edit-post/style-rtl.css	6.25 kB	0 B
build/edit-post/style.css	6.24 kB	0 B
build/edit-site/index.js	20.5 kB	0 B
build/edit-site/style-rtl.css	3.54 kB	0 B
build/edit-site/style.css	3.54 kB	0 B
build/editor/editor-styles-rtl.css	492 B	0 B
build/editor/editor-styles.css	493 B	0 B
build/editor/index.js	45.4 kB	0 B
build/editor/style-rtl.css	3.83 kB	0 B
build/editor/style.css	3.82 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	7.49 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	1.74 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.55 kB	0 B
build/is-shallow-equal/index.js	709 B	0 B
build/keyboard-shortcuts/index.js	2.39 kB	0 B
build/keycodes/index.js	1.85 kB	0 B
build/list-reusable-blocks/index.js	3.02 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.12 kB	0 B
build/notices/index.js	1.69 kB	0 B
build/nux/index.js	3.27 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.44 kB	0 B
build/primitives/index.js	1.34 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	13.7 kB	0 B
build/server-side-render/index.js	2.61 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.24 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.74 kB	0 B
build/warning/index.js	1.13 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

_{compressed-size-action}

[adamziel]

Blocked by #25625

[noisysocks]

No longer needed, see #23975 (comment).