Sequence diagram: activation bars and control flow blocks

[Copilot]

Adds support for Mermaid sequence diagram activation bars (activate/deactivate, +/- shorthand) and control flow blocks (loop, alt, par, critical, break).

Parser

• activate X / deactivate X standalone directives
• +/- activation shorthand on message targets: A->>+B: call activates B; B-->>-A: reply deactivates the source (B)
• loop/alt/par/critical/break [label] open a CF block; else/and/option are consumed as section separators without closing; end closes the innermost block
• Unified openBlocks stack replaces openRects — handles both rect bands and CF blocks
• Unclosed activations at end of input are auto-closed at the last message index

sequenceDiagram
    A->>+B: Request
    loop retry
        B->>C: Validate
        C-->>B: Result
    end
    alt success
        B-->>-A: OK
    else failure
        B-->>A: Error
    end

Layout

• LayoutSequenceActivationBars: places 8px-wide bars centered on participant lifelines; nesting level offsets bars to the right so stacked activations remain distinct
• CF block layout reuses the same start/end index → Y-range logic as rect bands

Rendering

• AppendActivationBars (new, called after AppendLifelines): renders narrow theme-colored <rect> elements on lifelines
• AppendSequenceCfBlock (new): renders UML combined-fragment style — dashed border, light fill, solid keyword tab at top-left (e.g. loop [Retry up to 3 times])
• Both group types are skipped in the main AppendGroup pass

Tests

• 27 new unit tests: standalone activate/deactivate, nested activations, all 5 CF keywords, separator handling
• 2 new E2E snapshot fixtures: mermaid-sequence-activation, mermaid-sequence-cf-blocks

[Copilot]

Pull request overview

Adds Mermaid sequence-diagram support for activation bars and UML-style control-flow (combined-fragment) blocks, spanning parser → layout → SVG rendering, with new unit + snapshot tests.

Changes:

• Parser: adds activate/deactivate, +/- shorthand handling, and CF blocks (loop/alt/par/critical/break + separators).
• Layout: computes activation bar geometry and reuses index-range → Y-range logic for CF blocks.
• Rendering: draws activation bars after lifelines and renders CF blocks as dashed bordered regions with a keyword tab.

Show a summary per file

File	Description
src/DiagramForge/Parsers/Mermaid/MermaidSequenceParser.cs	Parses activation directives/shorthand and CF blocks; unifies open block tracking.
src/DiagramForge/Layout/DefaultLayoutEngine.Sequence.cs	Lays out activation bars and positions CF-block groups using message index ranges.
src/DiagramForge/Rendering/SvgStructureWriter.cs	Renders CF blocks (combined fragments) and activation bars (dedicated pass).
src/DiagramForge/Rendering/SvgRenderer.cs	Integrates activation bar rendering into the sequence render pipeline.
tests/DiagramForge.Tests/Parsers/Mermaid/MermaidSequenceParserTests.cs	Adds unit coverage for activation and CF parsing behaviors.
tests/DiagramForge.E2ETests/Fixtures/mermaid-sequence-activation.*	Adds snapshot fixture for activation bar rendering.
tests/DiagramForge.E2ETests/Fixtures/mermaid-sequence-cf-blocks.*	Adds snapshot fixture for CF block rendering.

Copilot's findings

• Files reviewed: 7/9 changed files
• Comments generated: 3

[Copilot]

IsSequenceKeyword now treats control-flow separators/keywords (else/and/option, loop/alt/etc.) as hard sequence keywords. This can break multi-line note continuation: an indented note line beginning with e.g. and/else will stop being appended to the pending note, and then gets swallowed by the IsCfSeparator(line) branch—dropping user text. Consider excluding CF separators/keywords from the note-continuation keyword check (or only treating them as keywords when actually inside an open CF block).

[Copilot]

TryParseCfKeyword builds the keywords span from a collection expression on each call. Since this runs once per input line (and is also invoked from IsSequenceKeyword), it can introduce avoidable allocations/overhead. Consider hoisting the keyword set to a static readonly field (or similar) and matching against it.

[Copilot]

Sequence canvasWidth is finalized and stored before CF groups are laid out, but CF rendering adds a keyword/label tab whose width is based on estimated text width. For long CF labels the tab can exceed canvasWidth, and because SvgRenderer uses sequence:canvasWidth as the final SVG width, the tab/text can be clipped at the right edge. Consider expanding canvasWidth during layout based on the maximum CF tab width (and updating group widths accordingly).

[jongalloway]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

All three review points addressed in b3c22f7:

1. Note continuation + CF keywords: Removed IsCfSeparator and TryParseCfKeyword from IsSequenceKeyword. Words like else, and, loop are common enough to appear in note text; only hard sequence keywords (participant, Note, rect, activate, end, etc.) interrupt continuation. Added a regression test (Parse_MultilineNote_CfWordsInContinuationArePreserved).

2. TryParseCfKeyword allocations: Hoisted the keyword list to a static readonly string[] CfKeywords field.

3. CF tab label clipping: canvasWidth is now finalized after the CF block layout pass. If any CF block's keyword tab would overflow the canvas right edge, canvasWidth is expanded and a back-fill pass updates all group widths to the final value before storing sequence:canvasWidth.