docs: add change tracker architecture documentation (#9767)

Documents the ChangeTracker undo/redo system: how checkState() works, all automatic triggers, when manual calls are needed, transaction guards, and key invariants. Companion to #9623 which fixed missing checkState() calls in Vue widgets. ┆Issue is synchronized with this [Notion page](https://www.notion.so/PR-9767-docs-add-change-tracker-architecture-documentation-3216d73d365081268c27c54e0d5824e6) by [Unito](https://www.unito.io) --------- Co-authored-by: GitHub Action <action@github.com>
2026-03-12 16:40:05 +00:00 · 2026-03-12 03:03:07 -07:00
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+# Change Tracker (Undo/Redo System)
+
+The `ChangeTracker` class (`src/scripts/changeTracker.ts`) manages undo/redo
+history by comparing serialized graph snapshots.
+
+## How It Works
+
+`checkState()` is the core method. It:
+
+1. Serializes the current graph via `app.rootGraph.serialize()`
+2. Deep-compares the result against the last known `activeState`
+3. If different, pushes `activeState` onto `undoQueue` and replaces it
+
+**It is not reactive.** Changes to the graph (widget values, node positions,
+links, etc.) are only captured when `checkState()` is explicitly triggered.
+
+## Automatic Triggers
+
+These are set up once in `ChangeTracker.init()`:
+
+| Trigger                             | Event / Hook                                       | What It Catches                                     |
+| ----------------------------------- | -------------------------------------------------- | --------------------------------------------------- |
+| Keyboard (non-modifier, non-repeat) | `window` `keydown`                                 | Shortcuts, typing in canvas                         |
+| Modifier key release                | `window` `keyup`                                   | Releasing Ctrl/Shift/Alt/Meta                       |
+| Mouse click                         | `window` `mouseup`                                 | General clicks on native DOM                        |
+| Canvas mouse up                     | `LGraphCanvas.processMouseUp` override             | LiteGraph canvas interactions                       |
+| Number/string dialog                | `LGraphCanvas.prompt` override                     | Dialog popups for editing widgets                   |
+| Context menu close                  | `LiteGraph.ContextMenu.close` override             | COMBO widget menus in LiteGraph                     |
+| Active input element                | `bindInput` (change/input/blur on focused element) | Native HTML input edits                             |
+| Prompt queued                       | `api` `promptQueued` event                         | Dynamic widget changes on queue                     |
+| Graph cleared                       | `api` `graphCleared` event                         | Full graph clear                                    |
+| Transaction end                     | `litegraph:canvas` `after-change` event            | Batched operations via `beforeChange`/`afterChange` |
+
+## When You Must Call `checkState()` Manually
+
+The automatic triggers above are designed around LiteGraph's native DOM
+rendering. They **do not cover**:
+
+- **Vue-rendered widgets** — Vue handles events internally without triggering
+  native DOM events that the tracker listens to (e.g., `mouseup` on a Vue
+  dropdown doesn't bubble the same way as a native LiteGraph widget click)
+- **Programmatic graph mutations** — Any code that modifies the graph outside
+  of user interaction (e.g., applying a template, pasting nodes, aligning)
+- **Async operations** — File uploads, API calls that change widget values
+  after the initial user gesture
+
+### Pattern for Manual Calls
+
+```typescript
+import { useWorkflowStore } from '@/platform/workflow/management/stores/workflowStore'
+
+// After mutating the graph:
+useWorkflowStore().activeWorkflow?.changeTracker?.checkState()
+```
+
+### Existing Manual Call Sites
+
+These locations already call `checkState()` explicitly:
+
+- `WidgetSelectDropdown.vue` — After dropdown selection and file upload
+- `ColorPickerButton.vue` — After changing node colors
+- `NodeSearchBoxPopover.vue` — After adding a node from search
+- `useAppSetDefaultView.ts` — After setting default view
+- `useSelectionOperations.ts` — After align, copy, paste, duplicate, group
+- `useSelectedNodeActions.ts` — After pin, bypass, collapse
+- `useGroupMenuOptions.ts` — After group operations
+- `useSubgraphOperations.ts` — After subgraph enter/exit
+- `useCanvasRefresh.ts` — After canvas refresh
+- `useCoreCommands.ts` — After metadata/subgraph commands
+- `workflowService.ts` — After workflow service operations
+
+## Transaction Guards
+
+For operations that make multiple changes that should be a single undo entry:
+
+```typescript
+changeTracker.beforeChange()
+// ... multiple graph mutations ...
+changeTracker.afterChange() // calls checkState() when nesting count hits 0
+```
+
+The `litegraph:canvas` custom event also supports this with `before-change` /
+`after-change` sub-types.
+
+## Key Invariants
+
+- `checkState()` is a no-op during `loadGraphData` (guarded by
+  `isLoadingGraph`) to prevent cross-workflow corruption
+- `checkState()` is a no-op when `changeCount > 0` (inside a transaction)
+- `undoQueue` is capped at 50 entries (`MAX_HISTORY`)
+- `graphEqual` ignores node order and `ds` (pan/zoom) when comparing