Error in collapsible box layout

By default, this boxLayout will un-collapse and show a similar message. Did you try that? Does it not fit your requirements?

This is the default behavior of a box if there are validation errors in any of the fields in the contents of the box. Just make sure you have a validation or character limit and it should trigger this behavior. Here's an example:

a!boxLayout(
  label: "Box Layout",
  contents: {
    a!textField(
      label: "Test",
      characterLimit: 2,
      value: "abc"
    )
  }
)