Function scrollbar_joiner_fn 

pub fn scrollbar_joiner_fn(
    child: impl IntoUiNode,
    wgt_fn: impl IntoVar<WidgetFn<()>>,
) -> UiNode

P Widget function for the little square in the corner that joins the two scrollbars when both are visible.

This property sets the SCROLLBAR_JOINER_FN_VAR.