Enabled default block appender in gallery edit component

[dhruvikpatel18]

Followed PR of: #71204 (comment)

What?

This does seem like a good addition, similar to the Buttons block, and other design-related blocks such as Columns, Grid, and Group, which have an appender. The gallery block currently lacks one. so adding it would improve consistency.
If there’s enough consensus, should we raise a separate follow-up PR to address this?

By @im3dabasia

Screenshots or screencast

Before

After

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[Mamaduka]

Thanks for contributing, @dhruvikpatel18!

Can you include before/after screenshots for the PR.

[im3dabasia]

@dhruvikpatel18, when you have a moment, please rebase the PR with trunk so we can resolve the failing CI cases.

[Mamaduka]

@dhruvikpatel18, @im3dabasia, let's get more feedback before merging this. I checked some old issues, and it seems that having "Add new" in the toolbar was intentional. See: #21247 and #38036.

While consistency is the key, not all blocks are created equal, and we should also keep that in mind.

@glendaviesnz, I know it's been a while since the Gallery block refactoring, but do you have any additional details?

[jasmussen]

The motivation is not bad, but the solution should ideally be more holistical. I recently revisited this issue which showed the need to ponder these features somewhat more generically, whilst still considering their context. Some instincts from that:

• An inline plus button in row-based blocks (buttons, navigation, row)
• A slash command

The unclear piece is the "Add" button, which can take the shape of a plus, the text "Add", or a fixed position appender. We have a mix of all three: can they be combined to one or is it still valid? The benefit of a Plus/Add in the toolbar is that it could always insert after the selection, or before if you hold ⌥, and this could potentially replace/remove the need for a fixed position appender that currently exists on Group.

The nuance is, those all relate to either opening the block inserter directly, or inserting an empty paragraph to allow you to use the Slash command. In case of the gallery, it's different, because it does neither: it opens the media library modal. In that sense, I'd keep only the "Add" button for now, but encourage the more generic explorations for inserter improvements.