How to Create a Field where Editors can add a Soft Break to a String

I understand you need a field where editors can add soft breaks (line breaks without creating new paragraphs) to a string, and you've been experimenting with the block type but finding the UI too large for a single title.

Unfortunately, Portable Text doesn't natively support soft breaks (the <br> equivalent from HTML). When users press Shift+Enter in the Portable Text editor, it creates a new block rather than a soft break within the same block. This is a known limitation - there's a GitHub issue #2572 that discusses this exact problem with line breaks in Portable Text.

For setting a default block style to h1, you can use the initialValue property on your field. Here's how:

defineField({
  name: 'title',
  type: 'array',
  of: [
    {
      type: 'block',
      styles: [
        {title: 'H1', value: 'h1'}
      ],
      lists: [],
      marks: {
        decorators: [],
        annotations: []
      }
    }
  ],
  initialValue: [
    {
      _type: 'block',
      style: 'h1',
      children: [{_type: 'span', text: ''}]
    }
  ]
})

This sets the default style to h1 when a new document is created, though editors can still change it if you haven't removed the style selector entirely. You can learn more about this in the Initial Value Templates documentation.

However, I'd recommend a different approach for your use case:

Since you need soft breaks in what's essentially a title/heading field, consider using the text type instead of block. The text type supports multi-line strings and you can control the textarea size:

defineField({
  name: 'title',
  type: 'text',
  rows: 2 // Controls the visible height of the textarea
})

The text type gives you a simple textarea where Enter creates actual line breaks (soft breaks) in the string, which is exactly what you need. When rendering on your frontend, these line breaks will be preserved as \n characters that you can convert to <br> tags.

If you absolutely need the h1 semantic meaning stored with the content and still want soft breaks, you might need to implement a custom input component, but the text type with rows is the simplest and most practical solution for titles with line breaks.