Removing validation on change is not suitable for all scenarios, just this one. It is better to  code the cancel button to ignore validation.
In order to do this you need to intercept events in the capture stage, and then prevent the default behavior, before the textarea onchange() event has occurred.

Something like this for the cancel button:

didInsertElement: ->
    if @get('preventValidation')
      doCapture = true
      @$().get(0).addEventListener('mousedown', (e) =>
          e.preventDefault()
          e.stopPropagation()
          @triggerAction(true)
        , doCapture
      )



 where 'preventValidation' is a property on tent.Button which defaults to false.