FindFirstVisibleIndex typedef
Resolves a scroll offset to the first visible-position index whose row's bottom edge is past it.
The render object uses a binary search over per-nid offsets, with a special-case fast path when the bulk-only cumulatives are valid. The computer accepts that resolver as a callback rather than reimplementing it, so the bulk-fast-path knowledge stays inside the render object.
Implementation
typedef FindFirstVisibleIndex = int Function(double scrollOffset);