Fix hidden InnerBlocks.DefaultBlockAppender when block is deselected

[BugReportOnWeb]

What?

Closes #74060

Fixes a bug where using renderAppender={ InnerBlocks.DefaultBlockAppender } on a block causes the appender to be hidden and a empty dashed outline to appear when the block is deselected.

Why?

When a block author explicitly passes renderAppender={ InnerBlocks.DefaultBlockAppender } to InnerBlocks, the expectation is that the “Type / to choose a block” placeholder remains visible even when the block is not selected.

However, .block-editor-default-block-appender was previously hidden whenever its parent block was neither selected nor has a selected child, without accounting for cases where the default appender had been explicitly forced via renderAppender. As a result, the appender disappeared when the block was deselected.

How?

• In block-list/index.js, detect when CustomAppender is explicitly DefaultBlockAppender and pass an isDefaultAppender flag to BlockListAppender.
• In block-list-appender/index.js, convert that flag into an is-default-appender class on the appender wrapper element, providing a reliable CSS hook.
• In block-list/content.scss, scope the existing hiding rule so it excludes appenders explicitly rendered via renderAppender using .block-list-appender:not(.is-default-appender).

Testing Instructions

1. Open a post or page in the block editor.
2. Temporarily modify packages/block-library/src/group/edit.js (line 102) as following:

        } else if ( ! hasInnerBlocks ) {
                // When there is no placeholder, but the block is also empty,
                // use the larger button appender.
-               renderAppender = InnerBlocks.ButtonBlockAppender;
+               renderAppender = InnerBlocks.DefaultBlockAppender;
        }

3. Insert a Group block and select an option on the placeholder.
4. Click away to deselect the block.
5. Expected: The "Type / to choose a block" placeholder is visible.
6. Before this fix: The appender is hidden and a dashed outline appears.

Screenshots or screencast

State	Before	After
Selected
Unselected

Use of AI Tools

• ChatGPT for rephrasing and refining the PR description.
• GitHub Copilot for implementation-related assistance.

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Co-authored-by: BugReportOnWeb <devasheeshkaul@git.wordpress.org>
Co-authored-by: talldan <talldanwp@git.wordpress.org>

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