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/sourceSession.ts

@@ -76,6 +76,11 @@ export class SourceSession implements vscode.Disposable {
     this.post({ type: 'setView', view: 'source' });
   }
 
+  /** The page is being replaced by another one (the Graph page): Source is no longer shown. */
+  leave(): void {
+    this.setShown(false);
+  }
+
   /** Handles a Source message from the page; false for any other message. */
   handle(message: { type?: string }): boolean {
     if (!isSourceMessage(message)) return false;