src/vs/workbench/api/common/extHostWebviewPanels.ts/ExtHostWebviewPanels/$onDidDisposeWebviewPanel
class ExtHostWebviewPanels:	async $onDidDisposeWebviewPanel(handle: extHostProtocol.WebviewHandle): Promise<void> {
		const panel = this.getWebviewPanel(handle);
		panel?.dispose();

		this._webviewPanels.delete(handle);
		this.webviews.deleteWebview(handle);
	}