From “Developing for the JIRA project-centric view” in the Jira Server Developer Guide:
The most detailed and up to date documentation around this component can be found in the source code on
src/main/resources/static/components/subnavigator/Subnavigator.js
.
Awesome! Where do I find that source code? I understand it’s in the Jira projects plugin, but the source code for that plugin isn’t available to outside Atlassian, or is it?