THERE IS A NEW PREVIEW IN TOWN: https://gist.github.com/herooutoftime/59eebf7c0bbf1299410c
Up to now MODx lacks a real preview (which shows unsaved changes to a resource). This plugin is a workaround and simulates a preview behavior.
- Create a new resource, call it 'Preview'
- Create a new system setting with key 'previewer.resource_id' and save with above resource ID