[FanWu-ai]

Problem

TabsProcessor uses the transformed string index to calculate tab stops. String indexes and terminal columns differ after double-width and combining characters. For example, with tabstop=4, 界\tb currently expands to 界 b, placing b in terminal column 5 instead of 4.

Change

Track terminal columns separately from transformed string positions. Use the renderer's cached character widths, including control-character display mappings, to determine the next tab stop while keeping source/display mappings expressed as string indexes. Preserve zero-width escape fragments without counting or expanding their contents.

Regression coverage

Cover ASCII, wide characters before the first and repeated tabs, a wide-character sequence ending exactly on a tab stop, combining characters, C0/C1 control-character rendering, and zero-width escape sequences (including a literal tab inside an OSC sequence). Check the expanded text and both cursor-position mappings, including positions inside tab expansion.

AI-assisted implementation; the patch and test results were reviewed.

Validation

• Unmodified base: 156 tests passed; project lint, formatting, spelling, all three strict mypy platform checks and all pre-commit hooks passed.
• Regression against unmodified TabsProcessor: 8 failed, 1 passed.
• Fixed tree: 165 tests passed; ruff check, ruff format --check, typos, mypy --strict for win32/linux/darwin, all 12 prek hooks, and git diff --check passed.
• Independently reviewed, including additional actual Window rendering and mouse-mapping checks.
• Local runtime: Python 3.12.14 on Linux. Hosted CI has not yet run for this PR.

Commands run successfully against the final tree:

.venv/bin/coverage run -m pytest tests/
.venv/bin/ruff check .
.venv/bin/ruff format --check .
.venv/bin/typos .
.venv/bin/mypy --strict src/ --platform win32
.venv/bin/mypy --strict src/ --platform linux
.venv/bin/mypy --strict src/ --platform darwin
PREK_HOME=/tmp/prek-c .venv/bin/prek run --all-files
git diff --check

Packaging validation: .venv/bin/python -m build passed in an isolated build environment and produced both the sdist and wheel. Setuptools emitted deprecation warnings concerning upstream license classifiers; the build completed successfully.