src/vs/workbench/services/editor/common/editorGroupFinder.ts/isOpened
function isOpened(group: IEditorGroup, editor: EditorInput | IUntypedEditorInput): boolean {
	for (const typedEditor of group.editors) {
		if (typedEditor.matches(editor)) {
			return true;
		}
	}

	return false;
}