Depending on the underlying source of the bug there are other libraries involved, but the below should give you some hints:
https://docs.nextcloud.com/server/latest/developer_manual/index.html
https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/javascript-apis.html
Note that some front-end stuff (javascript) has to be compiled. Also I’ll disclaim that the front-end is not my area of expertise at all.