"At startup" vs "on startup"

I wrote this:

... show on startup of the application.

My colleague corrects me and says it should be:

... show at startup of the application

because at indicates a moment in time. However, when I google it I get more results on on.

What's correct/better?

At is commonly used to refer to specific moments in time. (In particular, while we'd use on for dates, we'd use at for times on a given date.)

On is commonly used to refer to events that computers react to (especially in software, but it would also cover hardware reacting to interrupts).

An application starting up is both a specific moment in time, and an event that software can react to. As such, both can be used.

I'd lean toward using at for the existing conditions at the time of the application starting (e.g. what environment variables are set, what other applications are running) and on for events which happen because of the applicaiton starting (whether planned events, or unplanned problems). However, I wouldn't consider either entirely wrong in either case.

In the technology world, it would be more appropriate to use "on startup", because "startup" is an "event" that the computer or application is responding too, rather than a point in time.

Similar examples would be "on click" or "on blur" or "on submit" . . . you would never use "at click" or "at blur" or "at submit".