[5.x]: Let live preview inject a custom event instead of only reloading the iframe

[Megafry]

What happened?

Description

Live Preview always reloads the iframe when the entry changes. That call is hardcoded:

cms/src/web/assets/cp/src/js/Preview.js

Lines 444 to 446 in 41b7fc1

	this.elementEditor.on('update', () => {
	this.updateIframe();
	});

A full iframe reload is heavy for a Nuxt front end. Each update boots the app again.

Please add an option to overwrite this handler, so a project can run its own code instead of updateIframe().

Steps to reproduce

1. Open Live Preview for an entry whose preview target is a Nuxt page.
2. Edit a field.
3. Craft runs updateIframe() and reloads the iframe.

Expected behavior

An option replaces that update handler. The project injects its own code and the iframe stays mounted:

this.elementEditor.on('update', () => {
  // project code, for example:
  iframe.contentWindow.postMessage(
    { name: 'cms.preview.updated', token },
    '*'
  )
})

Actual behavior

update always calls updateIframe(). There is no option to replace it.

Craft CMS version

5.11.3

PHP version

8.2.34

Operating system and version

Linux 5.14.0-162.18.1.el9_1.x86_64

Database type and version

MySQL 8.0.32

Image driver and version

Installed plugins and versions

No response

[linear-code[bot]]

CMS-2506