extensions/typescript-language-features/src/languageFeatures/signatureHelp.ts/TypeScriptSignatureHelpProvider/getActiveSignature
class TypeScriptSignatureHelpProvider:	private getActiveSignature(context: vscode.SignatureHelpContext, info: Proto.SignatureHelpItems, signatures: readonly vscode.SignatureInformation[]): number {
		// Try matching the previous active signature's label to keep it selected
		const previouslyActiveSignature = context.activeSignatureHelp?.signatures[context.activeSignatureHelp.activeSignature];
		if (previouslyActiveSignature && context.isRetrigger) {
			const existingIndex = signatures.findIndex(other => other.label === previouslyActiveSignature?.label);
			if (existingIndex >= 0) {
				return existingIndex;
			}
		}

		return info.selectedItemIndex;
	}