diff --git a/docs/graph.md b/docs/graph.md
new file mode 100644
index 00000000..29c7d24b
--- /dev/null
+++ b/docs/graph.md
@@ -0,0 +1,129 @@
+# Box Graph Internals
+
+## Transaction Model
+
+`BoxEditing.modify()` wraps all box graph mutations in a transaction:
+
+```
+beginTransaction()
+  modifier()          ← user code runs here (box creation, deletion, pointer changes)
+endTransaction()      ← deferred pointer notifications fire here
+validateRequirements()
+mark()
+notifier.notify()     ← BoxEditing subscribers notified (undo/redo state)
+```
+
+### Nested `modify()` calls
+
+When `modify()` is called while `#modifying` is true or the graph is in a transaction,
+it takes a shortcut path: it calls `this.#notifier.notify()` and then `modifier()` directly,
+without starting a new transaction. The box operations run inside the existing outer transaction.
+
+### Pointer Update Deferral
+
+During a transaction, pointer changes (e.g., `pointer.refer(target)`, `pointer.defer()`)
+are recorded in `#pointerTransactionState` but NOT applied immediately.
+
+At `endTransaction()`, the deferred pointer changes are processed:
+
+```typescript
+this.#pointerTransactionState.values()
+    .toSorted((a, b) => a.index - b.index)
+    .forEach(({pointer, initial, final}) => {
+        if (!initial.equals(final)) {
+            initial.ifSome(address => findVertex(address)?.pointerHub.onRemoved(pointer))
+            final.ifSome(address => findVertex(address)?.pointerHub.onAdded(pointer))
+        }
+    })
+```
+
+This means `pointerHub.onRemoved` / `onAdded` callbacks fire AFTER all mutations complete,
+during `endTransaction()`. Code subscribed via `pointerHub.catchupAndSubscribe()` (e.g.,
+`VertexSelection.#watch`) sees the changes only at this point.
+
+After pointer processing, `#inTransaction` is set to false. Then `#finalizeTransactionObservers`
+are executed (these can add more observers in a loop). Finally `onEndTransaction` fires.
+
+## Box Deletion and Cascade
+
+`box.delete()` computes dependencies via `graph.dependenciesOf(box)`:
+
+1. Follows **outgoing** pointers to downstream targets
+2. Follows **incoming** pointers that are `mandatory` to upstream boxes
+3. Collects all dependent boxes and pointers recursively
+
+Then:
+- All collected pointers are deferred (`pointer.defer()`)
+- All collected dependent boxes are unstaged (`box.unstage()`)
+- The root box is unstaged
+
+### Cascade Deletion via `Field.disconnect()`
+
+When a box is unstaged, its fields call `disconnect()`. For target fields with incoming pointers:
+
+```typescript
+disconnect(): void {
+    const incoming = this.pointerHub.incoming()
+    incoming.forEach(pointer => {
+        pointer.defer()
+        if (pointer.mandatory || (this.pointerRules.mandatory && incoming.length === 1)) {
+            pointer.box.delete()  // CASCADE: deletes the box that owns the mandatory pointer
+        }
+    })
+}
+```
+
+**Key implication**: If Box A has a `mandatory` pointer to Box B, deleting Box B
+will cascade-delete Box A within the same transaction.
+
+### SelectionBox Cascade
+
+`SelectionBox` has two mandatory pointers:
+- `selection` → the user's selection field
+- `selectable` → the selected vertex (e.g., a region box)
+
+When a region box is deleted, `disconnect()` on the region's field finds the SelectionBox's
+`selectable` pointer (which is mandatory) and cascade-deletes the SelectionBox.
+
+At `endTransaction()`, the SelectionBox's `selection` pointer fires `onRemoved` on the
+user's selection field, which triggers `VertexSelection.#watch.onRemoved`. This removes
+the entry from `#entityMap` and `#selectableMap`, and notifies `onDeselected` listeners.
+
+## VertexSelection and the `#watch` Mechanism
+
+`VertexSelection.#watch(target)` subscribes to the user's selection field's `pointerHub`:
+
+- **`onAdded`**: A new SelectionBox was created → adds entry to `#entityMap` and `#selectableMap`,
+  notifies `onSelected` listeners
+- **`onRemoved`**: A SelectionBox was deleted → removes entry from both maps,
+  notifies `onDeselected` listeners (which propagates to `FilteredSelection`)
+
+These callbacks fire during `endTransaction()`, NOT during `modifier()` execution.
+
+## Timing of Side Effects
+
+Within `BoxEditing.modify()`:
+
+| Phase | `#modifying` | `inTransaction()` | Pointer notifications | `#selectableMap` updates |
+|-------|-------------|-------------------|----------------------|------------------------|
+| Before `beginTransaction()` | true | false | No | No |
+| During `modifier()` | true | true | **Deferred** | No |
+| During `endTransaction()` | true | transitions to false | **Firing** | **Yes** |
+| After `endTransaction()` | true→false | false | Done | Done |
+| `notifier.notify()` | false | false | Done | Done |
+
+This means code running inside `modifier()` can safely iterate `#selectableMap`
+because it won't change until `endTransaction()`. But code triggered BY `endTransaction()`
+(via `onRemoved`/`onAdded` cascades, `finalizeTransactionObservers`, or `onEndTransaction`)
+runs AFTER the map has been modified.
+
+## Known Issue: Stale Deselection After Region Deletion
+
+When a region is deleted by the ClipResolver (e.g., during content-start trimming with overlap
+resolution), the cascade deletes the SelectionBox and cleans up `#selectableMap` at
+`endTransaction()`. If a reactive observer later tries to `deselect` the same region
+(e.g., from an animation frame callback), `#selectableMap.get()` throws "Unknown key"
+because the entry was already removed.
+
+Introduced by commit `608f0b48` ("prevent overlapping", Jan 26 2026) which added the
+overlap resolver to `RegionContentStartModifier.approve()`.
diff --git a/packages/studio/adapters/src/selection/VertexSelection.ts b/packages/studio/adapters/src/selection/VertexSelection.ts
index ba683acb..c4efe408 100644
--- a/packages/studio/adapters/src/selection/VertexSelection.ts
+++ b/packages/studio/adapters/src/selection/VertexSelection.ts
@@ -88,7 +88,8 @@ export class VertexSelection implements Selection<SelectableVertex> {
         }
         if (selectables.length === 0) {return}
         this.editing.modify(() => selectables
-            .forEach(selectable => this.#selectableMap.get(selectable.address).box.delete()), false)
+            .forEach(selectable => this.#selectableMap.opt(selectable.address)
+                .ifSome(entry => entry.box.delete())), false)
     }
 
     deselectAll(): void {