Fix tablist variants invisible under Prism (monochrome palette)

[Copilot]

All three tablist variants (cards, stacked, flat) rendered as white-on-white under Prism because ColorUtils.Vibrant(#FFFFFF, n) returns #FFFFFF unchanged, and render:noGradient=true was applied unconditionally, suppressing the border-gradient fallback.

Changes

• DefaultLayoutEngine.TabList.cs — all three layout methods (LayoutTabListCards, LayoutTabListStacked, LayoutTabListFlat):
  • ResolveEffectivePalette is only invoked when NodePalette is present (non-null, non-empty). When no palette is configured, the original [theme.NodeFillColor] fallback is preserved, keeping prior behavior for custom themes that rely on it.
  • When NodePalette is present but monochrome (e.g. Prism), ResolveEffectivePalette provides a chromatic fallback derived from BorderGradientStops or hue rotation.
  • render:noGradient=true is kept unconditional for stacked and flat — their band renderers consume tablist:tabStroke/contentStroke/accentColor metadata directly and never use the node-level gradient stroke, so gradient generation would be wasteful.
  • render:noGradient=true remains conditional for cards — its accent rect uses fill and stroke directly, so the gradient border is genuinely applied and visible.

// All three variants — null/empty palette still falls back to NodeFillColor
IReadOnlyList<string> effectivePalette;
if (theme.NodePalette is { Count: > 0 })
    effectivePalette = ThemePaletteResolver.ResolveEffectivePalette(theme);
else
    effectivePalette = [theme.NodeFillColor];

// Cards only — gradient suppressed unless using a monochrome fallback palette
bool useFallbackPalette = !ReferenceEquals(effectivePalette, theme.NodePalette);
if (!useFallbackPalette)
    titleNode.Metadata["render:noGradient"] = true;

// Stacked and flat — gradient always suppressed (stroke value is unused by these renderers)
titleNode.Metadata["render:noGradient"] = true;

• New E2E snapshot fixtures — conceptual-tablist-cards-prism, conceptual-tablist-stacked-prism, conceptual-tablist-flat-prism with generated baselines showing chromatic fills derived from Prism's gradient stops

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[Copilot]

Pull request overview

Fixes Conceptual tablist rendering under the Prism theme by ensuring tablist layouts use a chromatic palette when the theme palette is monochrome, and by adjusting gradient opt-out behavior. Adds E2E snapshot fixtures to lock in the corrected Prism output for all three tablist variants.

Changes:

• Update tablist layout variants (cards/stacked/flat) to use ThemePaletteResolver.ResolveEffectivePalette(theme) instead of directly consuming theme.NodePalette.
• Make render:noGradient=true conditional based on whether a fallback palette was used.
• Add three new Prism-focused E2E fixtures (+ baselines) for tablist cards/stacked/flat.

Reviewed changes

Copilot reviewed 4 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/DiagramForge/Layout/DefaultLayoutEngine.TabList.cs	Switch tablist palette selection to effective palette; make gradient opt-out conditional.
tests/DiagramForge.E2ETests/Fixtures/conceptual-tablist-cards-prism.input	New Prism tablist “cards” fixture input.
tests/DiagramForge.E2ETests/Fixtures/conceptual-tablist-cards-prism.expected.svg	Baseline snapshot for Prism tablist “cards”.
tests/DiagramForge.E2ETests/Fixtures/conceptual-tablist-stacked-prism.input	New Prism tablist “stacked” fixture input.
tests/DiagramForge.E2ETests/Fixtures/conceptual-tablist-stacked-prism.expected.svg	Baseline snapshot for Prism tablist “stacked”.
tests/DiagramForge.E2ETests/Fixtures/conceptual-tablist-flat-prism.input	New Prism tablist “flat” fixture input.
tests/DiagramForge.E2ETests/Fixtures/conceptual-tablist-flat-prism.expected.svg	Baseline snapshot for Prism tablist “flat”.

Comments suppressed due to low confidence (3)

src/DiagramForge/Layout/DefaultLayoutEngine.TabList.cs:357

• In the flat variant, the tablist renderer (AppendTabListFlatBand) always renders stroke="none" for the accent line and title bar, and does not use the node-level stroke value produced by AppendGradientDefs. With render:noGradient now conditional, fallback palettes will cause per-node gradient <defs> to be emitted even though they can’t be used by this renderer, increasing SVG size unnecessarily. Consider keeping render:noGradient for the flat layout, or extending the flat renderer/metadata to apply the border gradient if that’s desired.

            titleNode.Label.FontSize = titleFontSize;
            titleNode.Label.FontWeight = "bold";
            titleNode.Label.Color = titleBarTextColor;
            if (!useFallbackPalette)
                titleNode.Metadata["render:noGradient"] = true;

src/DiagramForge/Layout/DefaultLayoutEngine.TabList.cs:224

• ResolveEffectivePalette(theme) is now used even when theme.NodePalette is null/empty. Previously TabList used theme.NodeFillColor in that case; switching to a derived 8-color palette changes output for themes without an explicit palette. If that behavior change isn’t intended, keep the NodeFillColor fallback for empty palettes and reserve ResolveEffectivePalette for monochrome-but-present palettes (e.g., Prism).

        var effectivePalette = ThemePaletteResolver.ResolveEffectivePalette(theme);
        bool useFallbackPalette = !ReferenceEquals(effectivePalette, theme.NodePalette);

        for (int i = 0; i < titleNodes.Count; i++)
        {
            var titleNode = titleNodes[i];
            double rowY = pad + titleOffset + i * (bandHeight + rowGap);
            string catFill = effectivePalette[i % effectivePalette.Count];

src/DiagramForge/Layout/DefaultLayoutEngine.TabList.cs:340

• ResolveEffectivePalette(theme) is now used even when theme.NodePalette is null/empty. That removes the previous single-color fallback to theme.NodeFillColor, and can change TabList styling for themes that intentionally omit NodePalette. Consider keeping the NodeFillColor fallback for empty palettes, and using ResolveEffectivePalette only when an explicitly provided palette is monochrome/unusable.

        var effectivePalette = ThemePaletteResolver.ResolveEffectivePalette(theme);
        bool useFallbackPalette = !ReferenceEquals(effectivePalette, theme.NodePalette);

        double curY = pad + titleOffset;

        for (int i = 0; i < titleNodes.Count; i++)
        {
            var titleNode = titleNodes[i];
            int catIdx = GetMetadataInt(titleNode.Metadata, "tablist:categoryIndex");
            var items = itemsByCategory.TryGetValue(catIdx, out var list) ? list : [];
            int itemCount = items.Count;

            string catFill = effectivePalette[i % effectivePalette.Count];
            string titleBarFill = ColorUtils.Vibrant(catFill, 2.0);

[Copilot]

ResolveEffectivePalette(theme) is now used even when theme.NodePalette is null/empty. That changes TabList behavior from the previous fallback of using theme.NodeFillColor for all categories, which can surprise custom themes that intentionally rely on NodeFillColor when no palette is provided. Consider preserving the prior behavior for the null/empty palette case (use NodeFillColor), and only invoking ResolveEffectivePalette when NodePalette is present but monochrome (or otherwise deemed unusable).

This issue also appears in the following locations of the same file:

• line 217
• line 327

[jongalloway]

@copilot apply changes based on this feedback

[Copilot]

Fixed in 272b1f6. All three variants now only call ResolveEffectivePalette when NodePalette is present (non-null, non-empty) — if no palette is set, they fall back to [theme.NodeFillColor] exactly as before, preserving behavior for custom themes that rely on that path. Additionally, render:noGradient=true is now unconditional for stacked and flat since their band renderers use tablist:tabStroke/contentStroke/accentColor metadata directly and never consume the node-level gradient stroke; the conditional remains only on cards, which does use fill/stroke in its accent rect.

[Copilot]

In the stacked variant, the tablist renderer (AppendTabListStackedBand) does not use the node-level stroke value produced by AppendGradientDefs; it uses tablist:tabStroke / tablist:contentStroke instead. Making render:noGradient conditional here will therefore emit per-node <defs> (and compute gradients) that are unused in the final SVG for fallback palettes. Consider either keeping render:noGradient enabled for the stacked layout, or updating the stacked renderer/metadata to actually apply the gradient stroke when that’s the intent.

This issue also appears on line 353 of the same file.

Suggested change

	if (!useFallbackPalette)
	titleNode.Metadata["render:noGradient"] = true;
	titleNode.Metadata["render:noGradient"] = true;

[jongalloway]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

These changes were already applied in 272b1f6 — render:noGradient=true is now unconditional for stacked and flat (their renderers use tablist:tabStroke/contentStroke directly, not the node-level gradient stroke), and conditional only for cards. All three variants also preserve the [theme.NodeFillColor] fallback when NodePalette is null/empty.