fix: restore legacy Latest Posts grid columns - #85258

What?

Closes #85242

Restores the legacy default column count for Latest Posts blocks saved before the layout-support migration. Legacy grid content such as:

<!-- wp:latest-posts {"postLayout":"grid"} /-->

now renders columns-3 again, so the front end displays a three-column grid instead of a single stacked column.

Why?

#77989 moved Latest Posts to layout support and removed postLayout and columns from block.json. The old columns attribute had "default": 3, so grid blocks that used the default column count were saved with only "postLayout":"grid", with no columns key.

Before 7.1, block registration supplied columns => 3, and the render callback output columns-3. That default no longer exists, and the server fallback now resolves to null:

$column_count = $layout['columnCount'] ?? ( $attributes['columns'] ?? null );

The editor still falls back to 3 in edit.jsx and in the block deprecation, so editors see a three-column grid while the front end renders one column. Server rendering does not run editor deprecations, and saving a post without editing the block does not migrate the stored attributes.

How?

Restore the legacy default only when no layout attribute exists:

// Legacy content did not serialize the `columns` attribute because it
// defaulted to 3. Restore that default when no layout attribute exists.
$default_column_count = empty( $layout ) ? 3 : null;
$column_count         = $layout['columnCount'] ?? ( $attributes['columns'] ?? $default_column_count );

This keeps the following behavior unchanged:

• Legacy grid blocks with an explicit columns value keep that value.
• Legacy list-layout blocks do not receive grid column classes.
• Modern layout-support blocks keep their layout.columnCount behavior, including grids that intentionally omit columnCount.

No content migration or re-save is required; the fallback happens at render time. Added PHPUnit regression coverage for the legacy default, explicit column counts, list layout, migrated layout content, and modern layout grids.

Screenshots or screencast

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Use of AI Tools

AI assistance: Yes

Tool(s): OpenAI Codex
Used for: Reproducing the regression, checking the WordPress 7.0.4 and 7.1.3 render paths, drafting the PHP fix and PHPUnit regression tests, and drafting this PR description. I have reviewed and manually tested the code.