Fix: Content block disappears from List View when changing page slug with "Show template" enabled

[Mustafabharmal]

What?

Fixes #74884

The Content block disappearing from List View when changing a page's URL slug while "Show template" is enabled on block themes.

Why?

When editing a page with "Show template" enabled and changing the URL slug:

• The Content block would disappear from the Document Overview (List View)
• This only occurred on block themes like Twenty Twenty-Five

Root Cause:
The getTemplateId() selector in core-data uses the page slug to determine which template to display. When the slug changed, the selector would:

1. Immediately look for a slug-specific template (e.g., page-new-slug)
2. If not found, would temporarily return undefined
3. This caused the Editor component to re-render without a template
4. The re-render cleared the blocks from List View, losing the Content block

How?

Modified getTemplateId() in packages/core-data/src/private-selectors.ts to implement proper fallback logic:

Before: Attempted to find slug-specific template in a single call, which could return undefined
After: Two-step approach:

1. Try slug-specific template first (e.g., page-about, single-post-hello-world)
2. If not found or slug unavailable, immediately fall back to generic template (e.g., page, single-post)

This ensures the function always returns a valid template ID, preventing undefined states that cause the Content block to disappear.

Testing Instructions

1. Use a block theme (e.g., Twenty Twenty-Five)
2. Edit any page/post
3. Enable "Show template" from the Options menu (⋮)
4. Add some content to the page
5. Open Document Overview (List View)
6. In the sidebar, change the page's URL slug
7. Content block should remain visible in List View
8. Page content should remain editable

Screenshots or screencast

Before:

Screen.Recording.2026-01-23.at.5.09.45.PM.mov

After:

Screen.Recording.2026-01-23.at.5.12.08.PM.mov

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