Qt Quick MultiEffect Example: Testbed - Slider does not work as expected

Description: The slider doesn't work as expected. Will bounce back up and prevent the user from accessing the lower part of the options if not all fields are closed.

Steps to reproduce:

1. Open the Multi effect test bed.
2. Open one or two different filed as to make it necessary to slide to go downwards. 
3. Try to slide down using the mousewheel to open the very last field named "mask"

Expected behavior: The sliding movement should work normally no matter how many fields are opened.

Observed behavior: If even one field is open, will create issues when trying to slide downwards. The sliding will "bounce back"upwards.

Special notes: Use the mousewheel to slide.