In Shopify's theme "Dawn," section titles (headings) generally do not allow line breaks, but if you absolutely need to add a line break, you can do so by editing the code and changing the schema.
Example of setting to allow line breaks to be entered in items where line breaks are not allowed, such as the headings of each section of the theme "Dawn"
[(For the Dawn theme) Setup procedure] Change the `type` of the relevant schema item from `inline_richtext` to `richtext`
