Fix cursor drift when a wrapped line starts on an exact wrap boundary

[gyanu2507]

With wrap_lines=True, get_height_for_line treated text that fills the window width exactly as still sitting on the current row. _scroll_when_linewrapping measures the text before the cursor, so an exact fill means the cursor is already on the next visual row. The window then under-scrolls, the wrapped cursor cell is never painted, and it looks like the cursor drifted — usually when a space becomes the first character of a wrapped line. Arrow keys recalculate the position and snap it back.

This counts that extra row when measuring up to the cursor (both the fast path and the get_line_prefix path).

Fixes #2071