Allow all entry sources to be searched/filtered when limiting sources on an Entries field

[mijewe]

When configuring an Entries field's "Sources" setting, you can select which sources entries can be selected from.

When "All" is checked, the entry selection modal shows an "All entries" item in the sidebar that lets you search and filter across every source at once

When "All" is unchecked and only a subset of sources is selected instead, that "All entries" option disappears from the modal. The sidebar then only lists the individually selected sources, with no way to search or filter across all of them together.

This is a problem because the client might not remember which section an entry is in. As a developer, my only option is to give access to all sources but this isn't the desired behaviour.

This was originally raised back in 2018 (#2822) but never picked up, and the underlying limitation is still present in current Craft versions. The use case hasn't gone away: you still need to limit sources to keep users away from entries used purely for relational data, but that shouldn't mean losing the ability to search across the sources you've allowed.

Expected behavior
Even when a specific subset of sources is selected, the modal should still offer an "All entries" option that searches and filters across that whole selected subset, not just across a single source at a time.

Actual behavior
No combined search/filter view is available when more than one source is selected; users must click into each source individually to find an entry.