This is the most approachable actively maintained open source project I could find: 

https://github.com/Hufe921/canvas-editor

OnlyOffice is AGPL but they use a canvas: 

https://github.com/ONLYOFFICE

Collabora Online also uses canvase and is licensed under MPL 2: 

https://github.com/CollaboraOnline/online

SN release: allow browser translations and other bug fixes related to new editor

k00b

This is the most approachable actively maintained open source project I could find: https://github.com/Hufe921/canvas-editor

OnlyOffice is AGPL but they use a canvas: https://github.com/ONLYOFFICE

Collabora Online also uses canvase and is licensed under MPL 2: https://github.com/CollaboraOnline/online