A soft-wrapped row and the next row share a byte: the row's exclusive end
is the next row's start. Up or Down with a goal column past the target
row's width put the cursor on that byte, so it drew on the wrong row. Up
made no progress and Down skipped a row.
`row_with_end` tells a soft wrap from a hard break (byte-adjacent rows
are soft). `move_to_display_col_on_line` takes the row and its `RowEnd`
and clamps a soft row to its last grapheme, or to the start of an element
that closes the row. A hard-break row still reaches its newline.
`clamp_to_line` now cuts on graphemes, not chars, so a ZWJ cluster stays
whole.
Kigi differs from upstream's premise that elements wrap whole: with no
display override in the buffer, `wrapped_lines` uses plain textwrap and
an element can span rows. Snapping to such an element's start would
leave the target row (Down from byte 0 stuck at 0), so an element that
began on an earlier row keeps the boundary `display_col_to_buffer_pos`
chose, as before this change.
`wrapped_navigation_with_zwj_graphemes` asserted the bug: Up from the
end stayed on the last row (byte 22). It now expects row 1's cluster
start (byte 11), as upstream does.
Tests: upstream's four cases, the row-spanning element, and a seeded
property test (500 cases): each Up or Down moves exactly one row. The
property test fails with the clamp disabled.
Ported from upstream (grok-build ratatui-textarea/textarea.rs, 1.0.28).
ZacharyZhang-NY
443de79e2f