QtQuickShaderEffect-Example Dragging is only possible from the top left image

Description:

Changing the image is only possible when sliding from the top left one.

Steps to reproduce:

1.Open the qtquick shader effect 

2.Drag the top left image to see the next available one.

3.Repeat the operation dragging any other image (will not work)

Expected behavior:

Dragging to see the next image should work on all images.

Observed behavior:

Only the top left image can be dragged to see other images.

Special notes:

There should be some indication that only this image can be drag to see the others

or
It should be possible to drag on all images to see the next one.