[Ak4shh]

Fixes #1649.

Rectangle annotations omitted /C, which viewers can interpret as a transparent border. Rectangle now defaults to a black border and accepts a keyword-only border_color parameter. Passing None preserves the previous behavior of omitting /C. Interior color remains independently configurable.

Added tests for the default border and PDF round trips with black, red, and omitted border colors. Updated the annotation guide, including the spelling of interior_color.

Validation: 41 annotation tests passed; Ruff and focused mypy passed. PyMuPDF rendering confirmed visible borders for black and red and no border when the color is omitted. Acrobat and browser viewers were not checked.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.65%. Comparing base (9808f6b) to head (08552c3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4297   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files          59       59           
  Lines       11962    11964    +2     
  Branches     2270     2271    +1     
=======================================
+ Hits        11801    11803    +2     
  Misses         84       84           
  Partials       77       77           

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[PJBrs]

Would it be useful to have the new Color class in this PR? See here:

https://github.com/py-pdf/pypdf/blob/main/pypdf/generic/_color.py

[stefan6419846]

I do not agree that we should use a black border by default. Instead, we should provide a way for the user to pass a border color and default it to the old behavior (to retain backwards-compatibility).

[stefan6419846]

Would it be useful to have the new Color class in this PR?

This would require us to refine the public API for the Color class in a way that only the relevant functionality is exposed publicly and each change would require a deprecation process.

Does the class really have any benefits over passing hex-based strings here?

[Ak4shh]

Done. border_color now defaults to None, so /C is omitted as before.