Description

When a block type has margins, defined either through theme.json or global styles, they can be overridden by layout styles e.g. the constrained layout which removes the start and end margins on the first and last child elements respectively.

When the block list appender is inserted within a container with such a layout, the style removing the last child's margin now targets the appender instead of the last block causing a layout shift

Step-by-step reproduction instructions

1. Navigate to Appearance > Editor > Global Styles > Blocks > Cover
2. Apply vertical margins to Cover blocks
3. Add a Cover block within a Group block to a page or post
4. Note the Group block's default constrained layout removes the Cover block's margins
5. Select the Group block and see the bottom margin suddenly applied

Screenshots, screen recording, code snippet

Environment info

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Please confirm that you have searched existing issues in the repo.

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Please confirm that you have tested with all plugins deactivated except Gutenberg.

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