feat: warning about excessive refinement from gap meshing (FXC-3919)

[dbochkov-flexcompute]

Warn if automatically calculated dl from gap meshing is less than 0.1 x (current min mesh size). Threshold (0.1) is somewhat random, we could adjust if needed

---

Note

Adds a warning during auto grid generation to flag potentially excessive refinement from gap meshing.

• Introduces warning when LayerRefinementSpec.dl_min_from_gaps is < 0.1 × the smallest lateral grid size
• Extracts magic number 0.45 to DL_MIN_FROM_GAPS_FRACTION and adds GAP_REFINEMENT_WARNING_THRESH
• Replaces inline 0.45 * min_gap_width with DL_MIN_FROM_GAPS_FRACTION * min_gap_width
• Adds tests covering: very small gap (warning), larger gap (no warning), and dl_min_from_gap_width=False (no warning)
• Updates CHANGELOG.md under Unreleased

^{Written by Cursor Bugbot for commit 9786431. This will update automatically on new commits. Configure here.}

Greptile Overview

Greptile Summary

This PR adds a warning to alert users when gap meshing creates excessively fine mesh refinement. It introduces two new constants (DL_MIN_FROM_GAPS_FRACTION and GAP_REFINEMENT_WARNING_THRESH) and refactors existing code to use the named constant instead of a magic number.

Key changes:

• Extracts hardcoded value 0.45 into DL_MIN_FROM_GAPS_FRACTION constant
• Adds warning logic that triggers when calculated dl_min_from_gaps is less than 10% of the smallest lateral grid size
• Includes comprehensive test coverage with three scenarios: small gap (warning triggered), large gap (no warning), and disabled feature (no warning)

Minor issue:

• Missing CHANGELOG.md entry (required by custom instruction a109c62a-aa8c-4cc4-b515-0ad947c47929)

Confidence Score: 4/5

• This PR is safe to merge with minor style improvements recommended
• The implementation adds a helpful warning for users and includes thorough test coverage. The logic is sound and uses appropriate threshold values. The only issues are a missing CHANGELOG entry and a minor formatting inconsistency in the warning message (backticks vs single quotes). No functional bugs or security issues were detected.
• No files require special attention - the changes are straightforward and well-tested

Important Files Changed

Filename	Overview
tidy3d/components/grid/grid_spec.py	Added warning logic for excessive gap refinement with new constants; replaced magic number with named constant
tests/test_components/test_layerrefinement.py	Added comprehensive test for gap meshing warning with three test cases (small gap, large gap, disabled feature)

Sequence Diagram

sequenceDiagram
    participant User
    participant GridSpec
    participant LayerRefinementSpec
    participant Logger

    User->>GridSpec: Create simulation with gap meshing enabled
    GridSpec->>GridSpec: _make_grid_multiple_iterations()
    
    loop For each gap meshing iteration
        GridSpec->>LayerRefinementSpec: _resolve_gaps(old_grid, merged_geos)
        LayerRefinementSpec-->>GridSpec: (snapping_lines, gap_width)
        
        alt dl_min_from_gap_width enabled
            GridSpec->>GridSpec: Update min_gap_width
            
            alt gap_width < inf
                GridSpec->>GridSpec: Calculate lateral_grid_sizes
                GridSpec->>GridSpec: Find min_lateral_grid_size
                GridSpec->>GridSpec: Calculate dl_min_from_gaps = 0.45 * gap_width
                
                alt dl_min_from_gaps < 0.1 * min_lateral_grid_size
                    GridSpec->>Logger: log.warning("detected a very small gap width...")
                    Logger-->>User: Display warning about excessive refinement
                end
            end
        end
        
        GridSpec->>GridSpec: _make_grid_one_iteration() with dl_min_from_gaps
    end
    
    GridSpec-->>User: Return final grid with refined gaps

Loading

Context used:

• Rule from dashboard - Update the CHANGELOG.md file when making changes that affect user-facing functionality or fix bugs. (source)

[greptile-apps]

_{1 file reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[github-actions]

Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/grid/grid_spec.py (100%)

Summary

• Total: 11 lines
• Missing: 0 lines
• Coverage: 100%