Fix capacity overflow panic wrapping a wide char at narrow terminal width

[vjymisal0]

Bug

bat --terminal-width 1 --wrap character aborts with capacity overflow (exit 101) when a line contains a character wider than the terminal (a double-width CJK char or emoji) and a background is painted on the line (--highlight-line, or a theme/style that fills the row background). Reported in #3844.

Root cause

In InteractivePrinter::print_line (src/printer.rs), the character-wrapping branch accumulates cursor by each chunk's display width. When a single character is wider than the remaining max_width, it still gets pushed onto line_buf and its full width added to cursor at flush time — so cursor can end up greater than cursor_max (the terminal width). The end-of-line background fill then computes:

ansi_style.paint(" ".repeat(cursor_max - cursor))

cursor_max - cursor underflows (usize), producing a repeat count near usize::MAX, which aborts inside str::repeat.

This is the same defect class as the --style=snip width-1 panic fixed in #3804 — that fix clamps a different overflow site in the same function; this one is the last remaining unclamped subtraction of the same shape.

Fix

Clamp with saturating_sub, matching the pattern already used a few lines above (line ~789) for the no-wrap branch:

ansi_style.paint(" ".repeat(cursor_max.saturating_sub(cursor)))

When the cursor has overshot the terminal width, the background fill is now empty instead of underflowing — matching the "clamp to empty" behavior bat already uses elsewhere for this exact class of bug.

Testing

Reproduced the panic on current master before the fix:

$ printf '\U0001F4E6\U0001F4E6\n' | bat --highlight-line 1 --terminal-width 1 --wrap character --color always --paging never --theme OneHalfDark
thread 'main' panicked at src/printer.rs:961:49:
attempt to subtract with overflow

After the fix, the same command exits 0 with no panic.

Added a regression test, wide_char_wrap_at_terminal_width_one_with_highlight_does_not_panic, covering this exact case. Ran the full integration suite (cargo test --test integration_tests): 230 passed, 0 failed. Also ran cargo clippy --lib with no new warnings.

Added a changelog entry under Bugfixes per CONTRIBUTING.md.

Closes #3844

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.