Flow endpoint detach off a stock has no live preview (only valve moves)

[bpowers]

Problem

In src/diagram/drawing/Canvas.tsx deriveRenderState, dragging a flow endpoint (arrowhead/source) that is currently attached to a stock does not live-update the flow path. Only the flow valve moves during the drag; the flow line and arrowhead stay pinned to the stock until pointer-up, at which point the geometry snaps to a newly-created cloud at the release position (created by flow-attach.ts reattachEndpoint's newCloud branch).

This is the same class of bug as the recently-fixed cloud-endpoint case: for an along-axis (parallel) drag, the endpoint drag routes through applyGroupMovement -> UpdateFlow (drawing/Flow.tsx), which treats the drag as a valve slide rather than a reroute, so the path preview is stale.

Scope / why it wasn't fixed with the cloud case

The just-landed fix added a branch in deriveRenderState (Canvas.tsx around line 924-961) that calls growEndpointDrag (from src/diagram/flow-attach.ts) -> UpdateCloudAndFlow so the flow line, arrowhead, and valve all track the dragged endpoint. That branch is gated on endpointEl?.type === 'cloud'; a stock endpoint deliberately falls back to the old applyGroupMovement/UpdateFlow behavior. A live detach preview needs to synthesize a cloud that follows the cursor, which was out of scope for the reported cloud-only bug.

Why it matters

UX polish / consistency in the diagram editor. Detaching a flow endpoint off a stock feels broken relative to dragging a cloud endpoint: the user drags but the flow path doesn't follow, then it jumps on release. Low-to-medium priority.

Component(s) affected

• src/diagram/drawing/Canvas.tsx (deriveRenderState, the cloud-only endpoint-drag branch)
• src/diagram/flow-attach.ts (growEndpointDrag, reattachEndpoint)

Possible approach

Extend the growEndpointDrag/UpdateCloudAndFlow-based live-preview path to stock endpoints by synthesizing a transient cloud at the dragged endpoint (mirroring reattachEndpoint's newCloud branch) so the flow line tracks the cursor during a detach drag, matching the committed geometry and the cloud-drag feel.

Discovered

While fixing the cloud-endpoint live-update bug (the fix that added the growEndpointDrag branch in deriveRenderState).