Try: Fix appender margins again.

[jasmussen]

This is round 2 of #27274, which was reverted in #27309 because it broke the buttons block. This time I've tested more widely, including justifications. The purpose of the PR is the same:

The default block appender has an 8px margin above and below it. This margin usually collapses with adjacent margins that are almost always bigger.

However some themes zero out the top and bottom margins of the first and last blocks inside groups (such as Twenty Twenty One, see WordPress/twentytwentyone#859). When this is the case, the small 8px margin causes a jump

Before:

After:

[github-actions]

Size Change: +9 B (0%)

Total Size: 1.19 MB

Filename	Size	Change
build/block-editor/style-rtl.css	11.2 kB	+6 B (0%)
build/block-editor/style.css	11.2 kB	+3 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
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build/annotations/index.js	3.8 kB	0 B
build/api-fetch/index.js	3.42 kB	0 B
build/autop/index.js	2.84 kB	0 B
build/blob/index.js	665 B	0 B
build/block-directory/index.js	8.72 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-library/editor-rtl.css	8.86 kB	0 B
build/block-library/editor.css	8.87 kB	0 B
build/block-library/index.js	148 kB	0 B
build/block-library/style-rtl.css	8.27 kB	0 B
build/block-library/style.css	8.27 kB	0 B
build/block-library/theme-rtl.css	789 B	0 B
build/block-library/theme.css	790 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.06 kB	0 B
build/blocks/index.js	48.1 kB	0 B
build/components/index.js	172 kB	0 B
build/components/style-rtl.css	15.3 kB	0 B
build/components/style.css	15.3 kB	0 B
build/compose/index.js	9.95 kB	0 B
build/core-data/index.js	14.8 kB	0 B
build/data-controls/index.js	827 B	0 B
build/data/index.js	8.98 kB	0 B
build/date/index.js	11.2 kB	0 B
build/deprecated/index.js	768 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.95 kB	0 B
build/edit-navigation/index.js	11.1 kB	0 B
build/edit-navigation/style-rtl.css	881 B	0 B
build/edit-navigation/style.css	885 B	0 B
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build/edit-post/style.css	6.4 kB	0 B
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build/edit-site/style.css	4.06 kB	0 B
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build/edit-widgets/style.css	3.13 kB	0 B
build/editor/editor-styles-rtl.css	476 B	0 B
build/editor/editor-styles.css	478 B	0 B
build/editor/index.js	43.3 kB	0 B
build/editor/style-rtl.css	3.85 kB	0 B
build/editor/style.css	3.85 kB	0 B
build/element/index.js	4.63 kB	0 B
build/escape-html/index.js	735 B	0 B
build/format-library/index.js	6.86 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	2.27 kB	0 B
build/html-entities/index.js	623 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	698 B	0 B
build/keyboard-shortcuts/index.js	2.54 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.1 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	5.32 kB	0 B
build/notices/index.js	1.82 kB	0 B
build/nux/index.js	3.42 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.43 kB	0 B
build/priority-queue/index.js	791 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/reusable-blocks/index.js	2.92 kB	0 B
build/rich-text/index.js	13.4 kB	0 B
build/server-side-render/index.js	2.77 kB	0 B
build/shortcode/index.js	1.69 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	2.84 kB	0 B
build/viewport/index.js	1.86 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.22 kB	0 B

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