Writing flow: mark shift+click as a multi-selection gesture

[t-hamano]

What?

Cherry-picks #80286 into wp/7.1 to verify that it fixes the e2e failure on the branch.

Why?

Since the backport of #80687, the e2e job fails consistently on wp/7.1:

Details

 1) [webkit] › specs/editor/various/multi-block-selection.spec.js:1680:3 › Multi-block selection (@firefox, @webkit) › shift+click multi-selection › should grow and shrink a full selection with shift+arrow 

Error: expect(received).toMatchObject(expected)

    - Expected  -  0
    + Received  + 22

      Array [
        Object {
    +     "attributes": Object {
    +       "height": "100px",
    +     },
    +     "clientId": "842d183c-f6e5-4dee-8b7c-983801510802",
    +     "innerBlocks": Array [],
    +     "isValid": true,
          "name": "core/spacer",
        },
        Object {
    +     "attributes": Object {
    +       "height": "100px",
    +     },
    +     "clientId": "3184f1be-10d1-4029-ae26-c50f5c73584f",
    +     "innerBlocks": Array [],
    +     "isValid": true,
          "name": "core/spacer",
    +   },
    +   Object {
    +     "attributes": Object {
    +       "content": "a paragraph",
    +       "dropCap": false,
    +     },
    +     "clientId": "6702e1dc-9cda-408d-bd7b-3b263df5ea80",
    +     "innerBlocks": Array [],
    +     "isValid": true,
    +     "name": "core/paragraph",
        },
      ]

    Call Log:
    - Timeout 5000ms exceeded while waiting on the predicate

      1699 | 			await expect
      1700 | 				.poll( multiBlockSelectionUtils.getSelectedBlocks )
    > 1701 | 				.toMatchObject( [
           | 				 ^
      1702 | 					{ name: 'core/spacer' },
      1703 | 					{ name: 'core/spacer' },
      1704 | 				] );
        at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/various/multi-block-selection.spec.js:1701:6

The selection grows to three blocks with Shift+ArrowDown, but Shift+ArrowUp does not shrink it back to two. The run before that commit was green.

#80286 is the only writing-flow related PR from the same series that was not backported — #80287, #80374, #80396, #80462, #80549 and #80651 are all on the branch. It was likely missed because it does not carry a backport label.

Testing Instructions

If the analysis is correct, all CI checks on this PR should pass, in particular the e2e test above.

Use of AI Tools

The regression was investigated with Claude Code. The PR contains no AI-authored code — it is an unmodified cherry-pick of an existing commit.

[github-actions]

Size Change: -82 B (0%)

Total Size: 7.74 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	427 kB	-82 B (-0.02%)

_{compressed-size-action}

[github-actions]

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Co-authored-by: ellatrix <ellatrix@git.wordpress.org>

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[ellatrix]

Thanks!