I would recommend the user be prompted to save when quitting Luban if they haven’t done so already.
It’s a sad lesson to learn the hard way.
I’m guessing here, but Luban looks like it’s either based on or incorporates LaserWeb as some of its foundational bits. LW is some useful Open Source software, but unfortunately it, too, has some … demanding UI.
This feels like something they can fix fairly easily, though - good job reporting it, sorry you lost work. 
Hi Tone, I’ve already forwared this request to our software team. Sorry about your work.
I didn’t loose any work as I was just experimenting but I know someone will not and will loose work.