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.
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.