Is Nuxeo Studio open source?
yes, Studio is proprietary software.
If you worry about relying on a proprietary software for development while the Nuxeo Platform claims being Open Source, it's important to understand that Nuxeo Studio is "only" an additional service for Nuxeo customers: it is not mandatory and it's producing open code (mainly XML contributions).
Nuxeo Studio is a SaaS (Software as a Service) application allowing you to customize the Nuxeo products, from minor details to significant changes. It greatly lowers the development and maintenance costs by providing an advanced graphical tool, a bunch of automated checks, some templates and guidelines…, and automatic code upgrades following the Nuxeo Platform releases.
But at the end, what is produced by Nuxeo Studio and installed in your Nuxeo Platform is an open JAR which content may have been manually produced (thus requiring much more human time and platform knowledge). One last important point is that you do have access to your Studio project source code.