My friend texted me recently asking one of those good iPhone questions that is difficult to search for:
“Sometimes when I swipe my home button a few times, the screen drags itself down to a half screen. What is the point of that?”
The iPhone screen dragging itself down halfway is a feature of iOS for the iPhone called “Reachability”. The idea of the feature is to help people be able to reach and tap items on the top half of the screen when using the iPhone with one hand. Double-tap the home button and the screen contents slide down halfway; double-tap the home button again and the contents slide back up.
On an iPhone X/XS/XR, slide down at the bottom of the screen to activate reachablility and bring the top half of the screen down.
Reachability was added when the iPhone 6 and iPhone 6 Plus came out, with the larger screens. If you don't like Reachability sliding your screen down, you can fix it by turning Reachability off in the iPhone settings app:
Open the Settings app.
Select General > Accessibility.
Turn off the switch for Reachability.