[tom_]

An error about the app being damaged specifically can stem from the contents of the .app folder being modified after it was signed with the codesign tool. You can use codesign --verify (consult the man page) to get some info about why it's being considered damaged.

I've got this wrong in the past by adding the README too late in the process. Once I'd fixed that, the reason macOS gives for not opening my app became that Apple can't check it for malicious software. Much better... I think? The right click/open workaround does work.