Trix's `<trix-editor>` doesn't support the [form][] property like
`<textarea>` or other form fields.
For example, consider the following HTML and event listener:
```html
<form action="/articles" method="post">
<textarea name="content"></textarea>
<button type="submit">Save</button>
</form>
<script>
addEventListener("keydown", ({ key, metaKey, target }) => {
if (target.form && key == "Enter" && (metaKey || ctrlKey)) {
form.requestSubmit()
}
})
</script>
```
The `target` (an instance of `HTMLTextAreaElement` relies on the
[HTMLTextAreaElement.form][] property for access to its associated
`<form>`. While it's usually equivalent to `target.closest("form")`,
that isn't always the case. Declaring a `[form]` attribute with another
`<form>` element's `[id]` value can associate a field to a `<form>` that
is _not an ancestor_. That means that the event listener from above
would continue to work with this HTML:
```html
<textarea name="content" form="new_article"></textarea>
<!-- elsewhere -->
<form id="new_article" action="/articles" method="post">
<button type="submit">Save</button>
</form>
```
Unfortunately, if the `<textarea>` element were replaced with a
`<trix-editor>`, the event listener's reliance on accessing the form as
a property would break, since the `<trix-editor>` custom element doesn't
declare that property. There is currently a pull request
([basecamp/trix#899][]) to add support for accessing the `form` as a
property of the `<trix-editor>` element.
The [feedback][] provided on that pull request suggests that we
implement the `form` property by delegating to the `<input
type="hidden">` element. Currently, `<input type="hidden">` elements
constructed by Action Text helpers cannot declare the `[form]`
attribute.
This commit adds support by special-casing the `options[:form]` key
within `ActionText::TagHelper#rich_text_area_tag`.
[form]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLTextAreaElement#properties
[basecamp/trix#899]: https://github.com/basecamp/trix/pull/899#discussion_r618543357
[feedback]: https://github.com/basecamp/trix/pull/899#discussion_r618543357
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