If you look at the source file (https://github.com/coronalabs/framework-widgets-legacy/blob/master/widget_assets/graphics/TexturePacker/Android/slider_handle.png) you can see that the original image is flat at the top. This makes its way into the image sheets and I’m currently looking at a flat-topped slider in a test build on my Android phone.
I filed a bug report, but is there a workaround for this?