Currently, the collapsed state of any item in the blueprint tree is stored in egui's short-term cache. This has several limitations:

1. The cache is indexed by widget IDs which are unstable (at least by default). This means that any external change leading to a change in ID (e.g. the item is moved elsewhere in the hierarchy) invalidates the cache, leading to the collapsedness to potentially change. This happens when drag-and-dropping space view across containers.
2. It's currently difficult to externally control the current collapsed-ness state

Solving (2) is needed at least for:

• uncollapsing the hierarchy when focusing on items: Double-clicking an entity/instance/component anywhere should put it and its ancestors into focus _everywhere_ #5232
• dynamically uncollapse during drag-and-drop operations
• wide-ranging collapse/uncollapse context menu actions ("Collapse all"/"Expand all")

Possible approaches:

• Make widget ID predictable/stable (e.g. by deriving them from space view id, etc.).
  • How about DataResults?
  • Is there a public egui API to edit the collapsedness of a given widget ID?
• Take over egui and handle the collapsedness state on rerun side.
  • Using a Collapsed component?
  • Or using a short-lived ad hoc cache HashMap<re_viewer_context::Item, bool>?