[SoftTalker]

How does a real stopwatch work? Hint: it starts/stops on press, not release. So that's what the app should do, to map with prior expectations.

"Nobody uses real stopwatches anymore" might be somewhat true, but in athletics training they still do, and it just shows that it's not possible to adequately reproduce a thing on a touch screen and have it be as functional as the real item. A stopwatch app on a phone will always be a poor substitute for a real stopwatch.

[hananova]

Yes, but on a real stopwatch, or on a computer mouse, you can rest your finger on the button with almost but not quite enough force to actuate it, and activate it with a press.

On a touch screen you have to hover your finger over the button and move it down some indeterminate distance to actuate the button. The equivalent to the above example for a touch screen is to activate on release, as you can rest in a state “ready to click” just like on a physical button that way.

[IdiotSavage]

Interestingly the "activate on release" logic was implemented in computer GUIs long before touchscreens were common, used purely by mouse-click (a physical button).