When touch the slider while touching another area, the slider will move with the first touch

[sus-yoshikane-t]

Steps to reproduce

• Run on a device that supports multi-touch (such as a mobile device)
• Touch a part other than the slider (don't release it)
• Touch the slider handle (don't release it)
• The slider doesn't move even if you move the second touch (the touch
  other than the slider)
• The slider moves when you move the first touch (the touch on the slider handle)