Question: I'm looking for a customizable content editor that allows real-time collaboration and offline synchronization.

If you're looking for a customizable content editor with real-time collaboration and offline synchronization, Tiptap is a top contender. It's a headless content editor and a collaboration framework that can handle real-time collaboration, AI-generated content and version control. Its features include a Notion-like editor, AI copywriter, synced whiteboard, block editor and a commenting system with inline and document comments. It can use self-hosted and cloud storage and is very flexible for use cases like custom content sites, collaborative platforms and chat apps.

Another powerful option is Sanity, which offers a flexible, collaborative and scalable foundation for creating continuous digital experiences. Its extensible editing environment, called Sanity Studio, can be customized for each organization, and it offers APIs for integration with tools like Shopify and Mux. Features include real-time collaboration, GROQ for precise content querying, built-in AI Assist, and localization for consistent messaging. It's good for e-commerce, marketing sites, product management and mobile apps, and can be integrated with frameworks like React, Vue, Next.js and Gatsby.

Payload is another powerful option, an open-source, headless CMS and application framework written with TypeScript and React. It offers real-time collaboration, AI-powered content generation, visual editing and more than 20 field types. It also offers features for version management, draft previewing and single sign-on integration, making it good for agencies, marketers and developers. Payload is available under the MIT License with a free self-hosted option and scalable paid plans.

Bit.ai also offers a robust document collaboration option, particularly with its AI-powered features. It offers real-time collaboration, interactive living documents, AI-assisted writing and smart editor tools. The platform also includes features like trackable links, document tracking and powerful integrations, making it good for knowledge management and collaborative document creation. It offers flexible pricing plans, including enterprise options for more complex use cases.