val compute_offset : t ‑> get_float_elem_size:(unit ‑> float option) ‑> float
compute_offset
computes the extra offset from the edge of the viewport created by
having a floating element (header or first column).
get_float_elem_size
is a function that measures the relevant size of the floating
element if applicable (height for floating header, width for floating first column).
val is_floating : t ‑> bool
val px_from_edge : t ‑> int option