budget2.0/docs/specs/2026-09-30-session-undo-redo-design.md

Session undo and redo

The owner needs to reverse an accidental saved edit without reconstructing a budget from an export. History lasts for the current page session only; a reload starts with the saved dataset and empty history.

Behavior

• Every successful, changed mutate produces one undo step. Unchanged edits produce none. The last 50 prior datasets are retained.
• Undo and redo restore complete datasets, including IDs and dated rules, by writing through the store's existing serialization queue. A failed write changes neither the displayed dataset nor either history stack.
• A new successful edit after undo clears redo. A failed edit leaves redo available.
• Import and reset are destructive whole-dataset operations. Their existing backup-first flow remains mandatory. On success they clear both history stacks; on failure they leave history intact. Neither can be undone with the session controls.
• The header offers Undo and Redo buttons. Ctrl/Cmd+Z undoes and Ctrl/Cmd+Shift+Z or Ctrl/Cmd+Y redoes when focus is outside an editable control. Native text editing shortcuts remain native.
• A failed undo or redo shows its persistence error near the controls.

Implementation boundary

History belongs to src/store/snapshot.ts, where edits already serialize and only become visible after persistence succeeds. The UI reads stable capability state from the same store and invokes its methods; no domain or dataset schema change is needed. History is deliberately memory-only, so exports contain only the current dataset and IndexedDB needs no upgrade.