Problem:
Visual selection could end up in the wrong place after
nvim_buf_set_text or nvim_buf_set_lines. In some delete cases,
Visual.lnum was already clamped before the line shift happened, so the
adjustment got skipped.
Solution:
Split fix_cursor_cols into reusable fix_pos_col logic and reuse it
for Visual updates. Also adjust Visual.lnum before changed_lines so
the shift uses the original position before final clamping.
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