Fix Focus Loss on Tab/Shift+Tab After Changing Heading Level - #77959

What?

Closes #55745

This PR fixes a bug where keyboard focus is lost when navigating back to a Heading or Site Title block from the block toolbar using Tab or Escape, specifically after its heading level was changed.

Why?

When the heading level is changed (H2 -> H3), React completely unmounts the old element and mounts a new one with the updated tag. However, the internal focus state (lastFocus.current) still held a reference to the old, detached DOM element. Consequently, attempting to return focus to the block via .focus() failed silently, breaking keyboard accessibility.

How?

Added a .isConnected check for lastFocus.current. If the previously focused element is no longer connected to the DOM (i.e. it was replaced by a new heading tag), the logic falls back to querying the active block via its [data-block] attribute. This correctly targets the newly mounted element and reliably restores focus.

Testing Instructions

1. Open the block editor.
2. Add a Heading block.
3. Open the block toolbar.
4. Select a different heading level (e.g., change from H2 to H3).
5. Ensure focus can correctly be restored to the block (see keyboard instructions below).

Testing Instructions for Keyboard

1. Add a Heading block and type some text.
2. Press Shift + Tab to move focus from the block into the floating block toolbar.
3. Use the keyboard to navigate to the "Change level" button and change the heading level.
4. Press Tab to return focus to the block.
5. Expected: Focus successfully returns to the Heading block.

Screenshots or screencast

Before Fix :

After Fix :

Use of AI Tools

AI assitance used to draft PR description. all the changes are verified manually.