update available fields to include new fields ?

Problem/Motivation

Currently when a new field is added to an entity, the new field does not automatically show up in the Display Builder main UI (like it does in Layout Builder) but needs to be added manually from the library sidebar. This is perhaps not intuitive or newbie-friendly because with the sidebar collapsed, the newbie user might wonder "where is my new field? is there something wrong?", especially if they are coming from Layout Builder.

Current behaviour is because adding a new field would automatically update the saved display.

Proposed resolution

Pierre suggests that the main UI could reflect new fields automatically if the new field affected only the "instance data" (the entity with the live, not saved, display, InstanceInterface::setNewPresent()) and not the saved config or content (WithDisplayBuilderInterface::setSources()).