Function render 

pub fn render<V: 'static>(
    id: impl Into<ElementId>,
    doc: &Doc,
    layouts: &BlockLayouts,
    selection: Option<Selection>,
    dragging: bool,
    window: &mut Window,
    cx: &mut Context<'_, V>,
    on_pointer: impl Fn(&mut V, Pointer, &mut Context<'_, V>) + 'static,
) -> AnyElement

Render doc with selection painted in it, reporting what the pointer does.

dragging is the caller’s: a move only extends a selection that a press started, and which item that press landed in is not something one block of text can know.

Releasing is answered twice over — on the text and off it — because a drag that ends past the edge of a paragraph is the ordinary way to select to the end of one.