Table Of Contents: Include headings exclusively within the core/post-content block

[yogeshbhutkar]

What?

Closes #69359

This PR updates the logic to ensure the Table of Contents block only considers Heading elements within the core/post-content block. Previously, it included all headings on the page, including those in the Header and Footer, which could lead to unintended results.

Why?

Including Heading elements from outside the core/post-content block—such as those in the Header and Footer—in the Table of Contents block is not ideal, as it should strictly focus on the main content. The block should serve its intended purpose of summarizing the structure of the post by tabulating only the Heading tags used within the post content.

How?

The previous implementation used getClientIdsWithDescendants to get the client IDs of all the blocks in the editor. When the Show Template mode is toggled ( template-locked editing mode ), this selector literally returns all the blocks within the editor. That includes the Header, Footer, and the blocks within the Template. This behavior causes the bug mentioned above.

The current PR updates the usage to use getBlockOrder, which uses a recursive approach to exclusively get the nested blocks within the post-content using its Client ID. If the Client ID becomes undefined for some reason ( e.g. when editing mode is set to post-only ), in such a scenario, getBlockOrder gracefully falls back to return the top-level blocks.

Testing Instructions

1. Add Heading elements to the Header, Footer, or within the Template.
2. Create a new page.
3. Insert the Table of Contents (TOC) block.
4. Add Heading elements inside the post content.
5. Verify that the TOC block only includes headings from the post content and excludes those from the Header, Footer, or Template.
6. Toggle Show Template and confirm that the behavior remains consistent.

Screenshots

Before	After

Testing edge cases

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

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