Wire RadioButton stroke color (and document corner-radius/thickness limits)

[jonathanpeppers]

Routes IButtonStroke.StrokeColor (XAML's BorderColor on Microsoft.Maui.Controls.RadioButton) through Compose Material 3's RadioButtonColors ring slots, and explicitly holds back CornerRadius / StrokeThickness with a rationale comment.

What's wired

• ComposeBridges.RadioButton now accepts RadioButtonColors? colors = null and forwards to RadioButtonKt.RadioButton(colors: ...).
• New composer.RadioButtonColors(...) extension factory on IComposer mirrors CheckboxColors / SwitchColors — calls the parameterless RadioButtonDefaults.colors(composer, _changed) and threads non-null overrides through RadioButtonColors.copy(...) (the only four-long entry point that survives the binder strip from @JvmInline value class Color).
• RadioButtonColors added to the source-generator reference-type allow-list (ComposeReferenceTypes.cs) so [ComposeFacade] recognises it as an OptionalValue slot (auto-property + auto-mask). The generated RadioButton.Colors { get; set; } becomes a normal facade property.
• RadioButtonHandler adds [nameof(IButtonStroke.StrokeColor)] = MapStrokeColor and applies the colour to both selectedColor and unselectedColor so the user's chosen colour is visible regardless of check state — matches stock MAUI's UpdateStrokeColor semantic, which tints the ring drawable in every check state.

What's held back (intentionally)

IButtonStroke.StrokeThickness and CornerRadius are documented in the handler's mapper dictionary as deliberately unimplemented. Rationale (paraphrased from the new TODO block in RadioButtonHandler.cs):

Material 3's RadioButton draws a fixed 20.dp circle whose ring thickness and shape are baked into the composable's internal Canvas (drawCircle with a hard-coded RadioStrokeWidth = 2.dp); the public surface (RadioButtonColors) only exposes the ring colour, not its geometry. Honouring these would require replacing the call to RadioButtonKt.RadioButton with a hand-rolled Canvas { drawCircle / drawArc } composable, which loses M3's ripple, state-layer, focus indicator, and accessibility chrome. A CornerRadius override is also semantically off — XAML callers reaching for it on a RadioButton are typically trying to make a control that is no longer recognisable as a radio button.

Coverage

RadioButtonHandler: 36/39 (92%) → 37/39 (95%). Missing-key list shrinks from {CornerRadius, StrokeColor, StrokeThickness} to {CornerRadius, StrokeThickness}.

Repo-wide property-mapper coverage stays at 884/1224 (72.2%) since the new StrokeColor slot replaces an already-covered key in the regenerated docs/maui-coverage.md.

Sample

TogglesPage.xaml gains a fourth <RadioButton BorderColor="#D32F2F"> (Dragonfruit) in the existing radio group, so the change is visually verifiable on device.

Verified

• dotnet build src/Microsoft.AndroidX.Compose ✅ (0 errors)
• dotnet test src/Microsoft.AndroidX.Compose.SourceGenerators.Tests ✅ (155/155 pass)
• dotnet build src/Microsoft.AndroidX.Compose.Maui ✅ (0 errors)
• dotnet build src/Microsoft.AndroidX.Compose.Maui.Sample ✅ (0 errors)
• dotnet run scripts/maui-coverage.cs regenerates docs/maui-coverage.md cleanly.

[Copilot]

Pull request overview

This PR wires IButtonStroke.StrokeColor (MAUI's BorderColor on RadioButton) through Compose Material 3's RadioButtonColors ring slots, following the same colors-factory-and-copy pattern already established for CheckboxColors and SwitchColors. It also explicitly documents why StrokeThickness and CornerRadius are intentionally held back (Material 3's RadioButton composable hardcodes its ring geometry).

Changes:

• Adds ComposeExtensions.RadioButtonColors(...) factory on IComposer that calls RadioButtonDefaults.Instance.Colors(composer, 0) and applies overrides via RadioButtonColors.copy(...), plus registers RadioButtonColors in the source-generator's reference-type allow-list so [ComposeFacade] recognizes it as an OptionalValue slot.
• Updates ComposeBridges.RadioButton to accept and forward RadioButtonColors? colors = null to the underlying RadioButtonKt.RadioButton call.
• Adds MapStrokeColor in RadioButtonHandler to convert IButtonStroke.StrokeColor to a packed long? and apply it to both selectedColor and unselectedColor ring slots so the user-chosen color is visible in all check states.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Microsoft.AndroidX.Compose/ComposeExtensions.RadioButtonDefaults.cs	New IComposer.RadioButtonColors(...) extension mirroring RadioButtonDefaults.colors(), using the theme-defaults-then-copy pattern from CheckboxColors/SwitchColors.
src/Microsoft.AndroidX.Compose/ComposeBridges.cs	RadioButton bridge gains RadioButtonColors? colors = null parameter forwarded to the bound RadioButtonKt.RadioButton.
src/Microsoft.AndroidX.Compose.SourceGenerators/ComposeReferenceTypes.cs	Adds RadioButtonColors to the recognized reference-type list for [ComposeFacade] auto-property generation.
src/Microsoft.AndroidX.Compose.Maui/Handlers/RadioButtonHandler.cs	Adds MutableState<long?> _strokeColor, MapStrokeColor mapper, and BuildNode wiring that builds RadioButtonColors when stroke is set. Expands TODO comment for held-back properties.
src/Microsoft.AndroidX.Compose/PublicAPI.Unshipped.txt	Registers new RadioButton.Colors property and ComposeExtensions.RadioButtonColors(...) method.
src/Microsoft.AndroidX.Compose.Maui.Sample/Pages/TogglesPage.xaml	Adds a "Dragonfruit" RadioButton with BorderColor="#D32F2F" for on-device visual verification.
docs/maui-coverage.md	Regenerated: RadioButtonHandler coverage moves from 36/39 (92%) to 37/39 (95%); StrokeColor marked covered.