Function extend_span_drag 

pub fn extend_span_drag<T>(
    anchor_lo: T,
    anchor_hi: T,
    raw: T,
    unit_lo: T,
    unit_hi: T,
) -> (T, T)
where
    T: Ord + Copy,

Word/line-granularity drag extension — the shared rule behind double-click-then-drag in TextInput and the reader.

After a double-/triple-click snaps the anchor to a whole unit (anchor_lo, anchor_hi) (lo <= hi), a continuing drag must keep that unit inside the selection and grow by whole units. Given the boundary span (unit_lo, unit_hi) the pointer now sits in and its own position raw (to decide which side of the anchor it’s on), returns the (anchor, cursor) the selection should adopt.

T is whatever a selector indexes by — a byte offset (TextInput) or a (block, char) document position (the reader) — so both share one rule.