Add global diagram-theme directive with fence-level override precedence by Copilot · Pull Request #130 · jongalloway/MarpToPptx

Copilot · 2026-03-14T22:40:53Z

Adds a deck-level diagram-theme front matter directive so authors can set a preferred DiagramForge theme for all mermaid and diagram fences without repeating it per-fence. Fence-level theme: always wins.

Precedence

Fence-level theme: in embedded YAML front matter — unchanged, highest priority
Deck-level diagram-theme — injected when fence has no theme
Neither — DiagramForge falls back to default color mapping derived from the Marp slide theme

Example

---
diagram-theme: prism
---

---
theme: dracula   # overrides deck-level prism for this fence only
---
flowchart LR
  A --> B

Changes

Model / Parser

SlideDeck.DiagramTheme — new property stores the parsed directive value (whitespace-only → null)
MarpMarkdownParser — reads diagram-theme from front matter during deck parsing

Renderer

SlideRenderContext.GlobalDiagramTheme — carries the deck-level value through the render pipeline
InjectDiagramThemeIfNeeded — injects theme: X into the diagram source's YAML front matter before handing off to DiagramRenderer.Render(). Creates a minimal ---…--- block if the source has none; no-ops if a fence-level theme: is already present
FrontMatterBodyHasThemeKey — span-based line scanner (avoids string-split allocation) used by the above helper

Tests

4 parser tests: directive parsed correctly, absent → null, whitespace-only → null, stored in FrontMatter dict
5 renderer tests: deck-level applied to mermaid fence, deck-level applied to diagram fence, fence-level override produces different SVG than deck-level, no-regression without directive, front matter with other keys but no theme: receives injection while explicit theme: dracula is preserved

Docs / Sample

doc/marp-markdown.md — new table entry + "Diagram Theme Directive" section documenting precedence and syntax
samples/09-diagrams.md — adds diagram-theme: prism to front matter; labels slides showing deck-level inheritance vs. fence-level override

…edence Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

…eam variable Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

Copilot

Pull request overview

Adds a deck-level diagram-theme front-matter directive so authors can set a default DiagramForge theme for all mermaid / diagram fences, while keeping fence-level theme: as the highest-precedence override.

Changes:

Introduces SlideDeck.DiagramTheme and parses diagram-theme from front matter.
Threads the deck-level theme through the PPTX render pipeline and injects theme: ... into diagram fence YAML front matter when missing.
Adds parser/renderer tests plus updates documentation and the diagrams sample deck.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/MarpToPptx.Tests/PptxRendererTests.cs	Adds renderer tests validating SVG output and precedence behavior for deck-level diagram theming.
tests/MarpToPptx.Tests/ParserTests.cs	Adds parser tests ensuring `diagram-theme` is parsed, normalized, and stored correctly.
src/MarpToPptx.Pptx/Rendering/OpenXmlPptxRenderer.cs	Carries deck-level diagram theme through `SlideRenderContext` and injects it into diagram source when needed.
src/MarpToPptx.Core/Parsing/MarpMarkdownParser.cs	Reads `diagram-theme` from deck front matter and stores it on `SlideDeck`.
src/MarpToPptx.Core/Models/SlideDeck.cs	Adds the `DiagramTheme` property to the deck model.
samples/09-diagrams.md	Demonstrates deck-level default theme and per-fence overrides in the sample deck.
doc/marp-markdown.md	Documents the new directive, syntax, and precedence.

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tests/MarpToPptx.Tests/PptxRendererTests.cs

+        // ── diagram-theme directive tests ──────────────────────────────────────────
+
+        [Fact]
+        public void Renderer_GlobalDiagramTheme_MermaidFence_WithoutFenceLevelTheme_RendersSvg()
+        {
+                // A deck with diagram-theme: prism should apply the theme to mermaid fences without
+                // their own fence-level theme, and the diagram must render as an SVG picture shape.
+                using var workspace = TestWorkspace.Create();
+
+                var markdownPath = workspace.WriteMarkdown(
+                        "deck.md",
+                        """
+                        ---
+                        diagram-theme: prism
+                        ---
+
+                        # Mermaid Diagram
+
+                        ```mermaid
+                        flowchart LR
+                            A --> B
+                        ```
+                        """);
+
+                var outputPath = workspace.GetPath("deck.pptx");
+                RenderDeck(markdownPath, outputPath, workspace.RootPath);
+
+                using var document = PresentationDocument.Open(outputPath, false);
+                var slidePart = document.PresentationPart!.SlideParts.Single();
+
+                var pictures = slidePart.Slide!.Descendants<P.Picture>().ToArray();
+                Assert.NotEmpty(pictures);
+
+                var svgPart = slidePart.ImageParts
+                        .FirstOrDefault(p => string.Equals(p.ContentType, "image/svg+xml", StringComparison.OrdinalIgnoreCase));
+                Assert.NotNull(svgPart);
+
+                using var stream = svgPart!.GetStream();
+                var svg = new System.IO.StreamReader(stream).ReadToEnd();
+                Assert.Contains("<svg", svg, StringComparison.OrdinalIgnoreCase);
+
+                var textRuns = slidePart.Slide.Descendants<A.Text>().Select(t => t.Text).ToArray();
+                Assert.DoesNotContain(textRuns, t => t.StartsWith("Mermaid parse error:", StringComparison.Ordinal));
+
+                var validationErrors = new OpenXmlPackageValidator().Validate(document);
+                Assert.Empty(validationErrors);
+        }


tests/MarpToPptx.Tests/PptxRendererTests.cs

+
+                var textRuns = slidePart.Slide.Descendants<A.Text>().Select(t => t.Text).ToArray();
+                Assert.DoesNotContain(textRuns, t => t.StartsWith("Mermaid parse error:", StringComparison.Ordinal));
+


src/MarpToPptx.Pptx/Rendering/OpenXmlPptxRenderer.cs

+        if (source.StartsWith("---", StringComparison.Ordinal) &&
+            (source.Length == 3 || source[3] is '\n' or '\r'))
+        {
+            var openEnd = source.IndexOf('\n', 3);
+            if (openEnd < 0)
+                return source; // Opening delimiter with no newline — return unchanged.
+
+            var bodyStart = openEnd + 1;
+
+            // Search for the closing --- delimiter. Start from openEnd so the \n that
+            // terminates the opening delimiter line is included in the search window,
+            // which correctly handles an empty front matter block (--- immediately after ---\n).
+            var closeLineIdx = source.IndexOf("\n---", openEnd, StringComparison.Ordinal);
+            if (closeLineIdx >= 0)
+            {
+                var frontMatterBody = closeLineIdx >= bodyStart
+                    ? source[bodyStart..closeLineIdx]
+                    : string.Empty;
+
+                // If the fence already specifies a theme, it takes precedence — return unchanged.
+                // Use a span-based line scan to avoid allocating a split array.
+                if (FrontMatterBodyHasThemeKey(frontMatterBody))
+                {
+                    return source;
+                }
+
+                // Inject the global theme as the first key inside the existing front matter block.
+                return source[..bodyStart] + $"theme: {globalDiagramTheme}\n" + source[bodyStart..];


doc/marp-markdown.md

+````markdown
+```mermaid
+---
+theme: dracula
+---
+flowchart LR
+  A --> B
+````


samples/09-diagrams.md

 ## Mermaid With Dracula Theme

-This Mermaid diagram uses DiagramForge frontmatter to switch to the built-in Dracula theme and apply additional styling overrides.
+This Mermaid diagram uses DiagramForge frontmatter to override the deck-level `prism` theme with `theme: dracula`, plus additional styling options.


jongalloway · 2026-03-14T22:54:04Z

@copilot review comments

…oc closing fence, front matter spelling Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

Copilot · 2026-03-14T22:58:16Z

@copilot review comments

Addressed all five review comments in commit 6e95e41:

Test indentation — all 5 new renderer tests now use 4-space indent matching the rest of the file.
Nullable flow — all slidePart.Slide / sp.Slide accesses in the new tests now use the null-forgiving ! operator, consistent with the rest of the file.
InjectDiagramThemeIfNeeded trailing whitespace — replaced the literal position-based --- check with a trim-based first-line check (firstLine.Trim() == "---"). Added a FindClosingFrontMatterDelimiter helper that similarly trims each candidate closing line, so delimiters with trailing whitespace are correctly recognized.
Doc missing closing fence — added the missing closing ``` for the inner mermaid fence in the ````markdown` example block.
"frontmatter" → "front matter" — fixed all four occurrences in samples/09-diagrams.md.

Copilot AI and others added 3 commits March 14, 2026 21:13

Initial plan

4165846

feat: add global diagram-theme support with fence-level override prec…

95ebf7c

…edence Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

refactor: address code review — span-based theme key scan, rename str…

39ec366

…eam variable Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

Copilot AI assigned Copilot and jongalloway Mar 14, 2026

Copilot AI linked an issue Mar 14, 2026 that may be closed by this pull request

Add global diagram-theme support with fence-level theme override precedence #128

Closed

Copilot created this pull request from a session on behalf of jongalloway March 14, 2026 22:40 View session

Merge origin/main into PR #130

5e4ba32

jongalloway marked this pull request as ready for review March 14, 2026 22:47

jongalloway requested a review from Copilot March 14, 2026 22:47

Copilot started reviewing on behalf of jongalloway March 14, 2026 22:48 View session

Copilot AI reviewed Mar 14, 2026

View reviewed changes

Copilot started work on behalf of jongalloway March 14, 2026 22:54 View session

fix: address PR review — indentation, null-forgiving ops, --- trim, d…

6e95e41

…oc closing fence, front matter spelling Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

Copilot AI requested a review from jongalloway March 14, 2026 22:58

Copilot finished work on behalf of jongalloway March 14, 2026 22:58

jongalloway merged commit f1ce870 into main Mar 14, 2026
5 checks passed

jongalloway deleted the copilot/add-global-diagram-theme-support branch March 14, 2026 23:04

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Add global diagram-theme directive with fence-level override precedence#130

Add global diagram-theme directive with fence-level override precedence#130
jongalloway merged 5 commits intomainfrom
copilot/add-global-diagram-theme-support

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		var textRuns = slidePart.Slide.Descendants<A.Text>().Select(t => t.Text).ToArray();
		Assert.DoesNotContain(textRuns, t => t.StartsWith("Mermaid parse error:", StringComparison.Ordinal));

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Copilot AI commented Mar 14, 2026

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jongalloway commented Mar 14, 2026

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