What?

Tabs added beyond the initial two were silently deleted every time a saved page containing a Tabs block was opened for editing.

Closes #78241

Why?

TABS_TEMPLATE in tabs/edit.js specified nested inner blocks for core/tab-list (2 × core/tab) and core/tab-panels (2 × core/tab-panel). Because templateLock is 'all', the useInnerBlockTemplateSync hook always runs synchronizeBlocksWithTemplate on mount regardless of whether the block already has inner blocks.

synchronizeBlocksWithTemplate maps over the template entries, not the existing blocks, so it always produces exactly as many blocks as the template lists. A block saved with four tabs was reduced to two on every editor open, and the truncated state was persisted on the next save.

How?

Three minimal changes:

1. tabs/edit.js — TABS_TEMPLATE now names only the two structural child blocks (core/tab-list, core/tab-panels) without inner block entries. When synchronizeBlocksWithTemplate processes these template entries it passes undefined as the inner block template, which causes the function to return the existing inner blocks unchanged (the early if ( ! template ) { return blocks; } guard).

2. tab-panels/edit.js — TAB_PANELS_TEMPLATE is updated to create two labelled panels on initial insertion (matching the previous default). Because templateLock is false here, this template is only applied when the container is empty (new block), never to existing saved blocks.

3. tab-list/edit.js — Now initializes with two tab buttons by default, so the Tabs menu is always visible on first insert.

Testing Instructions

1. Insert a Tabs block (Gutenberg Experiments → Experimental Blocks must be enabled).
2. Add at least two additional tabs (3+ total) via the block toolbar.
3. Add content to each tab panel.
4. Publish / save the page.
5. Reload the editor.
6. Before fix: only the first two tabs remain. After fix: all tabs and their content are preserved.
7. Also verify that inserting a brand-new Tabs block still starts with two tabs.

Screenshots or screencast

Before:

After: