The viewer's ensure_cursor_visible scrolled the cursor to the
exact top or bottom edge of the viewport. Add a 2-line context
margin so the cursor sits 2 lines from the top (when scrolling
down) or 2 lines from the bottom (when scrolling up). Premium
polish — keeps the surrounding lines visible for context.
Tests: 1399 pass, zero warnings.
vasilito
914b2314e6