Description
handleMouse vi = Scroll + msg.Y - topPad + 1 has a spurious +1. Every left click maps one visual row too low; checkbox toggle only worked when the box was the last visual row (MoveToVisual clamp rescued it). Drop the +1.
Acceptance Criteria
- #1 Clicking a checkbox glyph on any row toggles that row's box
- #2 Regression test covers a mid-document checkbox with rows below it