Fix sidebar scroll issue on small viewports.

[jasmussen]

Fixes feedback from #20355 (comment).

The vh math was off, it counted 1 panel height and 1 adminbar height, but there are two panels headers to account for:

I wonder how long it's been like this.

[github-actions]

Size Change: -1 B

Total Size: 866 kB

Filename	Size	Change
build/edit-post/style.css	8.58 kB	-1 B

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.01 kB	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	104 kB	0 B
build/block-editor/style-rtl.css	10.5 kB	0 B
build/block-editor/style.css	10.5 kB	0 B
build/block-library/editor-rtl.css	7.68 kB	0 B
build/block-library/editor.css	7.68 kB	0 B
build/block-library/index.js	116 kB	0 B
build/block-library/style-rtl.css	7.49 kB	0 B
build/block-library/style.css	7.5 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	57.6 kB	0 B
build/components/index.js	191 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	5.76 kB	0 B
build/core-data/index.js	10.5 kB	0 B
build/data-controls/index.js	1.03 kB	0 B
build/data/index.js	8.22 kB	0 B
build/date/index.js	5.37 kB	0 B
build/deprecated/index.js	772 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	90.9 kB	0 B
build/edit-post/style-rtl.css	8.59 kB	0 B
build/edit-site/index.js	4.62 kB	0 B
build/edit-site/style-rtl.css	2.51 kB	0 B
build/edit-site/style.css	2.51 kB	0 B
build/edit-widgets/index.js	4.41 kB	0 B
build/edit-widgets/style-rtl.css	2.59 kB	0 B
build/edit-widgets/style.css	2.58 kB	0 B
build/editor/editor-styles-rtl.css	325 B	0 B
build/editor/editor-styles.css	327 B	0 B
build/editor/index.js	44.6 kB	0 B
build/editor/style-rtl.css	4.01 kB	0 B
build/editor/style.css	4 kB	0 B
build/element/index.js	4.45 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.6 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	1.92 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.48 kB	0 B
build/is-shallow-equal/index.js	710 B	0 B
build/keyboard-shortcuts/index.js	2.3 kB	0 B
build/keycodes/index.js	1.68 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	4.85 kB	0 B
build/notices/index.js	1.57 kB	0 B
build/nux/index.js	3.02 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	1.49 kB	0 B
build/priority-queue/index.js	879 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/rich-text/index.js	14.3 kB	0 B
build/server-side-render/index.js	2.54 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 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}

[youknowriad]

LGTM 👍