I have a textarea which is using Summernote and I set onChange event to it. I want to write test with RSpec + Capybara + Poltergeist to confirm the onChange event is working
Capybaras fill_in only works with html form elements. Since the JS version of your page is using a DIV with the contenteditable attribute as its text area to be filled #fill_in will not work. Instead you need to find the div and call #set on it directly. In your project the following should work
find('div[contenteditable]').set('This is awesome blog.')