Improvement Multi-Select UX for Relation Fields (hasMany: true)

[neltevreeke]

When using the Select component (specifically with relation fields where hasMany: true, the current behaviour closes the dropdown and clears the search input after each item is selected. This interrupts the flow of selecting multiple items, as the user has to reopen the dropdown and retype their search term repeatedly. Something you would not expect when a user typically needs to add more than one item with hasMany: true.

Steps to reproduce:

1. Create a collection with a relationship field configured with hasMany: true
2. Open the admin UI and navigate to a document with this field
3. Start typing to apply a search term value.
4. Click one of the results to select it.
5. Observe the dropdown closing and clearing the search term.

Expected behaviour:

For a better user experience, especially when selecting multiple related items:

1. The dropdown should remain open
2. The search input should retain its value
3. this allows users to select several items in succession of one another without having to click and apply the search term again.

Suggested fix:

When hasMany: true is set on a relationship field, update the Select component behaviour to:

• Prevent the dropdown from closing after a selection
• Keep the search input value intact

I believe this shouldn't be super hard to implement considering payload is using the react-select library and their component API already has configurations in place to make this possible, e.g. closeMenuOnSelect and blurInputOnSelect. This change would align the component's UX with more common patterns and make the admin experience significantly faster when working with large quantities of documents.