可以将ColumnSet的参考宽度设置为Auto，并将Action.ShowCard的width属性设置为"stretch"，这样ShowCard会继承ColumnSet的宽度。

示例代码：

{
  "type": "AdaptiveCard",
  "version": "1.0",
  "body": [
    {
      "type": "ColumnSet",
      "width": "auto",
      "columns": [
        {
          "type": "Column",
          "width": "stretch",
          "items": [
            {
              "type": "TextBlock",
              "text": "This is a ShowCard example"
            },
            {
              "type": "ActionSet",
              "actions": [
                {
                  "type": "Action.ShowCard",
                  "title": "Click me",
                  "card": {
                    "type": "AdaptiveCard",
                    "body": [
                      {
                        "type": "TextBlock",
                        "text": "This is a ShowCard"
                      }
                    ]
                  }
                }
              ]
            }
          ]
        }
      ]
    }
  ]
}