private void clampYTranslation(StackScrollState.ViewState childViewState,int childHeight){
  clampPositionToBottomStackStart(childViewState,childHeight);
  clampPositionToTopStackEnd(childViewState,childHeight);
}

private void clampYTranslation ( StackScrollState . ViewState childViewState , int childHeight ) { clampPositionToBottomStackStart ( childViewState , childHeight ) ; clampPositionToTopStackEnd ( childViewState , childHeight ) ; }
clamp the ytranslation both up and down to valid positions .
