If thats you, lets jump right in to permanently disable one-handed mode on iPhone.

So, what causes the annoyance?

Well, its in the way the Reachability is activated.

Choose Accessibility

On an iPhone with the Home Button, double-tap on the Home Button to activate the feature.

Considering how smooth sailing the gesture is, many users tend to trigger it accidentally.

Again, this Accessibility is well-thought-out and deserves praise.

turn off one handed mode on iPhone