ImGuiPaletteMapper makes selected text unreadable in all 38 themes (Text on TextSelectedBg ≈ 2:1), and slider grabs / plot lines barely show against FrameBg

[matt-edmondson]

What's wrong

ImGuiPaletteMapper chooses priorities that put several foreground/background pairs ImGui draws on top of each other at almost the same lightness. The worst case is text selection.

TextSelectedBg is mapped to (Alternate, High) (ThemeProvider.ImGui/ImGuiPaletteMapper.cs:94) and is fully opaque. In a focused InputText, ImGui fills the selection rectangle with that colour and then draws Text (Neutral, VeryHigh) on top of it. For comparison, ImGui's own default is translucent: (0.26, 0.59, 0.98, 0.35).

Contrast ratios from new ImGuiPaletteMapper().MapTheme(theme), measured over all 38 themes in ThemeRegistry.AllThemes at 9454d42:

Pair (foreground on background)	Mapping lines	Lowest	Median	Themes below threshold
Text on TextSelectedBg	:94, :44	1.45	1.97	38 of 38 below 3:1
SliderGrab (Primary MediumLow) on FrameBg (Neutral Low)	:70, :40	1.34	1.85	38 of 38 below 3:1
PlotLines (Alternate Medium) on FrameBg	:82, :40	1.69	2.38	36 of 38 below 3:1
Text on FrameBgActive (Neutral Medium)	:44, :42	2.37	3.27	38 of 38 below 4.5:1

Worst cases:

• VSCode Light: selected text is #000000 on #4a005e, a ratio of 1.45, so it effectively disappears.
• Everforest Light: selected text is #5c6a72 on #dc6902 (1.61), and the slider grab is #afc541 on #d8d8c8.
• Catppuccin Mocha: selected text is #cdd6f4 on #b787aa (2.06).

WCAG sets 4.5:1 as the minimum for body text and 3:1 for non-text UI parts. The only contrast test today, MapTheme_TextContrastsWithWindowBackground, only asserts that Text and WindowBg are not identical.

Suggested fix

• TextSelectedBg: map it to a low accent priority (VeryLow or Low, as Button does) and/or give it a translucent alpha, so Text keeps its contrast.
• SliderGrab and PlotLines/PlotHistogram: move them toward the high-visibility end, the way CheckMark uses VeryHigh.
• FrameBgActive: keep it near Low or MediumLow, so text inside an active frame stays readable.

Acceptance criteria

Add a test that iterates ThemeRegistry.AllThemes and asserts these minimums:

• Text on TextSelectedBg and on FrameBgActive: at least 4.5:1.
• SliderGrab, PlotLines and CheckMark on FrameBg: at least 3:1.

This is separate from #110, which covers ImGuiCol slots that are not mapped at all, and #114, which covers VeryHigh neutral tinting.

[matt-edmondson]

Triage

• Category: Bug (accessibility)
• Priority: Medium. Selected text is below 3:1 in all 38 built-in themes and effectively invisible in some, for example VSCode Light at 1.45:1. Slider grabs and plot lines are also below the WCAG 3:1 non-text minimum almost everywhere. Every consumer of ImGuiPaletteMapper is affected, but the UI remains usable.
• Area / suggested assignment: ThemeProvider.ImGui/ImGuiPaletteMapper.cs (mapping lines ~40, 42, 44, 70, 82 and 94).
• Duplicates: none. As the issue notes, it is distinct from ImGuiPaletteMapper leaves 13 ImGuiCol slots unmapped, so light themes keep dark defaults (white text cursor, navy dimmed tabs) #110 (unmapped slots) and A custom theme containing #ffffff (or #000000) gets a tinted, too-dark Neutral VeryHigh text colour, and can invert priority order: the chroma search starts from the out-of-gamut target #114 (VeryHigh neutral tinting). All three change the same mapping table, so a combined pass with the new contrast test would avoid re-tuning the table several times.
• In progress: no matching open PR.

Notes: The proposed contrast test over ThemeRegistry.AllThemes is the most valuable part of this. Land it first, marked expected-failing if necessary, so each mapping change can be measured against it. A translucent TextSelectedBg alpha, matching ImGui's own default of 0.35, is probably the smallest single change that fixes the worst pair.

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