`black==25.11.0` removing `%% [markdown]` comments

[jamesbraza]

Describe the bug

As of black==25.11.0 but not in black==25.9.0, it's removing extra code comments.

To Reproduce

Run black a.py on the below:

# %% [markdown]

# fmt: off
# fmt: on

It overzealously removes the top comment, leaving just:

# fmt: off
# fmt: on

Expected behavior

black to not remove the header comments, as they're part of Jupytext's percent format.

Environment

• Black's version: 25.11.0 (and current main branch at black==25.11.1.dev3+gf209c53a0)
• OS: macOS 15.6.1, build version 24G90
• Python version: Python 3.13.5 (main, Jul 10 2025, 15:26:00) [Clang 17.0.0 (clang-1700.0.13.5)]

Additional context

Coming from mwouts/jupytext#1467 (comment)

[HWKDS]

@jamesbraza i created a pr on this issue. #4845
it will help resolve this integration issue between Jupytext and Black.

[motalib-code]

@jamesbraza ---Issue #4843 is RESOLVED ✅

Black 25.11.0 was incorrectly removing # %% [markdown] comments.
The fix updates _handle_comment_only_fmt_block() in black/comments.py to preserve comments before # fmt: off.
All tests pass, and the reproduction case now works correctly — the # %% [markdown] comment is preserved.