0.9.3: Graph from Source on main.archify works again

On a main.archify page in Source, Graph built the Graph page but the
host held it back until Source closed, so nothing happened, and Model
then showed the held Graph page. Switching between the model and its
graph now leaves Source first, and the page forgets Source when Graph
replaces it, so Model on the Graph page comes back to the model.

src/views/diagramView.ts

@@ -165,6 +165,10 @@ class DiagramPreview implements vscode.Disposable {
 
   /** Switches a main.archify editor between its model page and the Graph page. */
   setGraph(on: boolean, query?: string): void {
+    // Leaving Source for another page: drop what it held, or that page would
+    // wait for a sourceClose that never comes and show up later instead.
+    this.heldHtml = undefined;
+    this.source?.leave();
     this.graph = on;
     this.graphQuery = query;
     const modes = this.graphModes();