What

The Design panel should display available navigation overlay patterns when editing navigation overlay template parts, but it's missing entirely. Only the "Content" section appears. This works correctly for headers and footers.

The patterns exist in lib/overlay-patterns.php and work via the block inserter, but they're filtered out from the Design panel.

Why

Navigation overlays are shipping in WordPress 7.0, and pattern discovery is essential for adoption. Without the Design panel, users can't easily discover available patterns, creating an inconsistent experience compared to other template parts.

This broke during experiment stabilization (#74968) when overlay patterns were assigned 'source' => 'core'. Since September 2023, patterns with source: 'core' are excluded from template part Design panels. This collision wasn't caught because no tests cover this scenario.

How

Navigation overlay patterns are marked 'source' => 'core' in lib/overlay-patterns.php, causing EXCLUDED_PATTERN_SOURCES to filter them. Possible approaches:

Adjust the source attribute

• Change to 'source' => 'plugin' or remove it entirely
• Needs coordination between Gutenberg and Core 7.0

Refine the filtering logic

• Special-case overlay patterns in filterOutExcludedPatternSources
• Consider exempting template-part-specific patterns

Input welcome from folks familiar with the pattern system.

User Stories

As a site builder, I want to see available navigation overlay patterns in the Design panel, so I can easily browse and select an overlay design.

As a theme author, I want the overlay editing experience to match headers and footers, so users understand how to customize template parts consistently.

Technical Context

Reproduction:

1. Site Editor → Patterns → Template Parts
2. Create template part with area "Navigation Overlay"
3. Check sidebar - Design panel is missing

Root cause: When stabilized, patterns changed from no source to `'source' => 'core'`:

// Before
register_block_pattern('gutenberg/navigation-overlay', [
    'blockTypes' => ['core/template-part/navigation-overlay'],
]);

// After - triggers filtering
register_block_pattern('core/navigation-overlay', [
    'blockTypes' => ['core/template-part/navigation-overlay'],
    'source' => 'core',
]);

Scope: Gutenberg trunk + WordPress Core 7.0 (backported via wordpress-develop#10844)

Related: #74968, #74069, #62034