Fix markdown preview not setting context properly on first creation

Fixes #50558

extensions/markdown-language-features/src/features/previewManager.ts

@@ -116,6 +116,8 @@ export class MarkdownPreviewManager implements vscode.WebviewPanelSerializer {
 			this.topmostLineMonitor,
 			this.contributions);
 
+		vscode.commands.executeCommand('setContext', MarkdownPreviewManager.markdownPreviewActiveContextKey, true);
+
 		return this.registerPreview(preview);
 	}