ADR-0089: Remember the unfolded tree; survive a deleted folder

• Status: Accepted
• Date: 2026-09-09
• Deciders: jka
• Decision: D89

Context

After ADR-0088 a reopened project came back with its tree collapsed. Two edge cases also needed answers: the project folder being deleted while rio runs, and the remembered folder being gone at the next launch.

Decision

• The unfolded set is stored in the project's workspace session (ADR-0031), beside the open files, not in prefs.json. It is project content, and storing it with the core means it also works with a remote core. workspace.save and workspace.get carry an expanded list, pruned of directories that no longer exist. It is saved on every fold and unfold. Sessions written before this change read as an empty set.
• A project folder deleted during a run closes the project. The files pane checks the root with one fs.list; on failure it clears the root and the unfolded set, shows the "no folder" placeholder without an error dialog, and forgets the reopen pointer if it named this root. A vanished subdirectory needs nothing, because its parent's listing no longer contains it.
• A remembered folder gone at launch is skipped and the pointer cleared.

Alternatives considered

Deleting the orphaned session file when a project is forgotten. Not done: sessions are keyed by path hash, so a recreated folder legitimately resumes, and deleting a non-current project's session would require the frontend to name a root other than the open one, which the workspace design forbids. The cost is one small file per project ever opened.

Consequences

• A project reopens with the tree as the user left it, locally and remotely.
• Deleting the project folder is handled quietly.
• A garbage collector for old session files would be a core-side addition if ever needed.