Editor: Reinitialize TinyMCE after meta box move

[sanketio]

Trac ticket

https://core.trac.wordpress.org/ticket/65958

What

Moving a meta box re-inserts its DOM subtree; when the box holds a wp_editor() TinyMCE editor, the re-inserted iframe is reset and TinyMCE keeps a stale reference, so the editor goes blank and the next Visual/Code switch throws.

postbox.js now fires the existing postbox-moved event on the Move up/down paths (it already fires on cross-area drag). A listener in editor.js re-initialises each editor in the moved box — save content, remove the stale instance, re-init in the same mode — keeping postbox.js editor-agnostic.

Test

• grunt jshint:core — 98 files lint free
• Register a wp_editor() meta box, type in Visual, Move up/down: editor keeps its content, getDoc() === iframe.contentDocument stays true, no console error
• After the move, switch Code then Visual: no setBaseAndExtent TypeError
• Same, in Firefox

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4.8
Used for: root-cause analysis, implementation, and local verification

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props sanketparmar.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.