Keep the dragged edge on screen while the palette picks its node

Releasing an edge over empty space opened the node palette but dropped the edge
drawing, so the connection being made was invisible exactly while its target was
being chosen. The finished drag is now held in `pendingDrop` and drawn like a
running one until the palette closes, by pick or by Escape.

The e2e check for the Escape path exposed a real defect underneath: `Popup.open()`
blurs the focused widget, `close()` did not. A field inside the popup therefore kept
the keyboard bridge's DOM focus after the popup was gone, and every later Delete or
Ctrl+Z went into that invisible input. The suite caught it one check later ("Delete
removes the node and its edge" found the node still there). `close()` now blurs,
mirroring `open()`.

Verified remotely on the Windows host: 287 editor checks and 24 jump-and-run smoke
checks, no failures.

src/editor/debugHook.ts

@@ -152,6 +152,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
         graphEditor.canvas.portGlobal(nodeId, port, isOutput),
       nodeAt: (nodeId: string) => graphEditor.canvas.nodeGlobal(nodeId),
       dropTarget: () => graphEditor.canvas.pendingDropTarget(),
+      danglingEdge: () => graphEditor.canvas.danglingEdgeVisible(),
       // The canvas, not the whole tab: a centre measured through the header sits above the
       // drawing area.
       center: () => centreOf(graphEditor.canvas),