If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Sounds like you have a great opportunity to collaborate on technical details, etc.. but still have separate projects. More power to you!
Now if you got more specific and the other author happened to have beat you in a student council election then I'd say it's probably time to forgive and forget.
Are you planning on making a GTK4 Lemmy frontend? That would be awesome!
I'm reminded of a time I found myself using an open source tool on github and finding it severely out of date on sources of information it was using to operate. I made a fork, spent a few hours updating, committed that code and put in a pull request with the original developer so they could merge it back into their original. 5 Years later, no response. 🤣
People abandon projects for various reasons or only work with the scarce free time they have. You may find someone interested in a healthy competition, but it might be more likely they back off when they see someone pick up the torch and do what they no longer can.
FOSS shit is built to be reproduced. if you wanted to fork somebody's project and build your own thing on top of it, that's in the spirit of open source. if they didn't want it to happen, they wouldn't use a FOSS license. if you can do that, then building your own thing that does similar stuff is certainly not a problem. FOSS isn't competitive, its collaborative. everybody who contributes is doing a good thing, even if that's by building more options for people.