[swf1996120]

Problem

This fixes #5536. Timestep.dimensions accepted a negative unit-cell length. The stored dimensions then produced a zero triclinic matrix and zero volume, silently turning invalid periodic geometry into a degenerate box.

Fix

Normalize the input to a six-element float array and reject negative values in the first three length fields. The existing all-zero no-unit-cell sentinel remains supported.

Tests

Added regression coverage for negative finite and negative-infinite lengths. Existing zero-box behavior remains unchanged.

This was found during a chemical-repository testing study using physical unit-cell validity as the domain-specific invariant.

[read-the-docs-community]

Documentation build overview

📚 MDAnalysis | 🛠️ Build #35508107 | 📁 Comparing d5e73e0 against latest (ae9ff4b)

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2 files changed

± index.html
± documentation_pages/analysis/wbridge_analysis.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (ae9ff4b) to head (d5e73e0).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5537      +/-   ##
===========================================
- Coverage    93.92%   93.91%   -0.01%     
===========================================
  Files          184      184              
  Lines        22661    22661              
  Branches      3223     3223              
===========================================
- Hits         21284    21283       -1     
- Misses         917      918       +1     
  Partials       460      460              

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[IAlibay]

@swf1996120 - we require everyone to fill in the PR template especially the AI use section and having the developer certificate of origin declaration.

The former is made clear in our AI policy: https://github.com/MDAnalysis/mdanalysis/blob/develop/AI_POLICY.md#0-ai-usage-must-always-be-declared

Given this failure to follow policy, and your excessive pull requests across the open source ecosystem (e.g. openmm/openmm#5559 (comment)), I am going to block and close this PR.