[Copilot]

Mermaid's rect rgb(...)...end / rect rgba(...)...end syntax was silently ignored. This adds full parse → layout → render support for coloured background bands in sequence diagrams.

Changes

Model

• Group: Added Metadata dictionary (Dictionary<string, object>) matching the existing Node/Edge pattern, enabling parsers to annotate groups without subclassing.

Parser (MermaidSequenceParser)

• Parses rect rgb(R,G,B) and rect rgba(R,G,B,A) opening lines and end closing lines using a stack (handles nesting).
• Each block maps to a Group with FillColor set and sequence:rectGroup=true / sequence:rectStartIndex / sequence:rectEndIndex stored in metadata.
• Blocks enclosing zero messages are dropped.

Layout (DefaultLayoutEngine.Sequence)

• After per-message sequence:messageY positions are finalised, rect-band groups receive X=0, Width=canvasWidth, and Y-bounds computed so adjacent bands share a midpoint boundary (firstMsgY − halfRow → lastMsgY + rowH − halfRow) with no overlap.

Renderer (SvgStructureWriter)

• AppendGroup detects sequence:rectGroup and delegates to a new AppendSequenceRectBand helper: a plain <rect> spanning the full canvas width.
• rgb() colors get fill-opacity="0.2"; rgba() colors use the embedded alpha directly.

Example input

sequenceDiagram
    participant A as Alice
    participant B as Bob
    A->>B: Initialise
    rect rgb(0,128,255)
        A->>B: Query
        B-->>A: Result
    end
    rect rgba(255,128,0,0.25)
        A->>B: Notify
        B-->>A: Ack
    end

Tests

• 9 new parser unit tests covering rgb/rgba parsing, metadata, index bounds, empty blocks, and message count.
• New snapshot fixture mermaid-sequence-rect with generated SVG baseline.

[Copilot]

Pull request overview

Adds end-to-end support for Mermaid sequence-diagram rect rgb(...) ... end / rect rgba(...) ... end background bands by extending the semantic model (Group metadata), parsing rect blocks into groups, laying them out after message positions are known, and rendering them as plain SVG <rect> elements.

Changes:

• Extend Group with a Metadata dictionary so parsers/layout can tag group instances without subclassing.
• Parse rect rgb(...) / rect rgba(...) blocks in MermaidSequenceParser into groups annotated with sequence:rect* metadata.
• Lay out and render rect-band groups in the sequence layout engine and SVG writer; add parser unit tests and an E2E snapshot fixture/baseline.

Show a summary per file

File	Description
tests/DiagramForge.Tests/Parsers/Mermaid/MermaidSequenceParserTests.cs	Adds unit tests validating rect parsing, metadata, and interaction with participant/message parsing.
tests/DiagramForge.E2ETests/Fixtures/mermaid-sequence-rect.input	New E2E input fixture covering both rgb() and rgba() rect bands.
tests/DiagramForge.E2ETests/Fixtures/mermaid-sequence-rect.expected.svg	New snapshot baseline verifying SVG output includes the background rect bands.
src/DiagramForge/Rendering/SvgStructureWriter.cs	Detects rect-band groups and renders them as simple semi-transparent <rect> elements.
src/DiagramForge/Parsers/Mermaid/MermaidSequenceParser.cs	Implements rect block parsing via a stack and attaches rect metadata to groups.
src/DiagramForge/Models/Group.cs	Adds Group.Metadata to match Node/Edge metadata patterns.
src/DiagramForge/Layout/DefaultLayoutEngine.Sequence.cs	Computes rect-band group bounds from per-message Y positions and canvas width.

Copilot's findings

• Files reviewed: 6/7 changed files
• Comments generated: 1

[Copilot]

SvgRenderer renders groups in a deterministic order using ThenBy(g => g.Id). With IDs like sequence:rect:0..10, lexicographic sorting will place ...:10 before ...:2, which can change z-order for overlapping/nested rect bands once there are >9 blocks. Consider zero-padding the numeric suffix (e.g., sequence:rect:0001) or storing a separate numeric metadata key for ordering.

[Copilot]

Fixed in cdfff2a by changing the group ID format to use D4 zero-padding (e.g., sequence:rect:0000, sequence:rect:0001, ..., sequence:rect:9999). This ensures ThenBy(g => g.Id, StringComparer.Ordinal) in SvgRenderer produces correct z-order for all realistic rect-block counts.

[jongalloway]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved in c517327. The conflict was in MermaidSequenceParserTests.cs where our rect block tests overlapped with the autonumber tests added in main — both test sections are now included.