Fixes#4870#4717#3260#4431#2718#4898#5036
- Fix the chat composer “phantom space” bug that appeared when
backspacing CJK (and other double-width) characters after the composer
got a uniform background in 43b63ccae89c….
- Pull diff_buffers’s clear-to-end logic forward to iterate by display
width, so wide graphemes are counted correctly when computing the
trailing column.
- Keep modifier-aware detection so styled cells are still flushed, and
add a regression test (diff_buffers_clear_to_end_starts_after_wide_char)
that covers the CJK deletion scenario.
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Co-authored-by: Josh McKinney <joshka@openai.com>
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