Figured out the way to do it. Instead of using a WKInterfaceTextField, I now use a button that sets a flag, then uses presentTextInputController to get the user's text input. In the willDisappear() function, check the flag and not stop my timer. Once the text input is done (or cancelled), unset the flag, so that when the user actually leave this screen, the timer will be stopped.

Unfortunately, this solution allows for double-tap on the screen, not the double-tap gesture with the fingers.