5N: focus box twitches in display (touch focus feature)

This is probably normal, but I thought I'd ask anyway.

When you touch the screen on the area you want to be in primary focus, the box goes where you touched, but when you remove your finger, the box shudders and shakes.

The box will stop shaking only when you set the focus (press shutter half way). Is this normal? It can be a problem sometimes because if you wait too long it will jump out of the chosen area.