RangeThumbSelector typedef
Decides which thumbs (if any) should be selected.
The default finds the closest thumb, but if the thumbs are close to each
other, it waits for movement defined by dx to determine the selected
thumb.
Override RangeSlider.thumbSelector for custom thumb selection.
Implementation
typedef RangeThumbSelector = Thumb Function(
TextDirection textDirection,
RangeValues values,
double tapValue,
Size thumbSize,
Size trackSize,
double dx
);