Fill missing MAUI property-mapper keys

[jonathanpeppers]

Drives every Compose-backed MAUI handler in docs/maui-coverage.md toward
100% property-mapper key coverage. Stacked on PR #269 originally; rebased
onto main after that PR merged.

Headline numbers

Metric	Before	After
Handlers we override	24 / 43 (55.8%)	24 / 43 (55.8%)
Property-mapper keys covered	830 / 1224 (67.8%)	883 / 1224 (72.1%)
Leaves category coverage	92%	98%
Containers category coverage	89%	95%
Pages / Navigation	86% (yellow)	100% (green)

Handler count is unchanged on purpose — opening new red ❌ handlers
(FlyoutViewHandler, NavigationViewHandler, ShapeViewHandler,
WebViewHandler, …) is explicitly out of scope per the request.

Per-handler — what got wired

Pages / Navigation

• PageHandler — Title ✅ via Activity title forwarding (with TODO
  for Shell-owned case).

Containers

• LayoutHandler — ClipsToBounds ✅ via Modifier.Clip.
• RefreshViewHandler — IsRefreshEnabled ✅ (gates the
  onRefresh callback + visually dims when disabled).
• BorderHandler — Shape ✅ (alias mapping to the same slot
  StrokeShape already wires).

Leaves

• DatePickerHandler — CharacterSpacing ✅, IsOpen ✅
  (two-way bind to the modal dialog).
• TimePickerHandler — CharacterSpacing ✅, IsOpen ✅.
• IndicatorViewHandler — HideSingle ✅, MaximumVisible ✅.
• LabelHandler — CharacterSpacing, LineHeight, Padding,
  TextDecorations ✅ (4 keys).
• PickerHandler — CharacterSpacing, HorizontalTextAlignment,
  IsOpen, Items ✅ (4 keys).
• ButtonHandler — CharacterSpacing, CornerRadius, Font,
  Padding, Source (image inside button), StrokeColor,
  StrokeThickness ✅ — 7 new keys, 100%.
• EntryHandler / EditorHandler / SearchBarHandler — refactored
  onto Compose's OutlinedTextField(MutableState<TextFieldValue>)
  overload via a TextFieldValueState : MutableState<TextFieldValue>
  subclass that intercepts every write to enforce
  ITextInput.MaxLength and mirror text + caret/selection back to
  the MAUI virtual view. New keys wired across the three:
  CharacterSpacing, HorizontalTextAlignment,
  IsSpellCheckEnabled, IsTextPredictionEnabled (collapsed →
  autoCorrectEnabled), MaxLength, PlaceholderColor,
  ReturnType (Entry/SearchBar), CursorPosition,
  SelectionLength, ClearButtonVisibility (Entry trailing X),
  SearchIconColor + CancelButtonColor (SearchBar trailing X).

Deliberate TODOs (left unmapped on purpose)

These remain yellow in the coverage report instead of being wired to
no-op mappers — per the request, partial coverage is fine, lying
coverage is not. Each carries a // TODO: comment in source naming
the blocker:

• BorderHandler (88%) — StrokeDashOffset, StrokeDashPattern,
  StrokeLineCap, StrokeLineJoin, StrokeMiterLimit. Dashed strokes
  need a custom DrawScope-based modifier; Compose's stock
  Modifier.Border doesn't expose dash configuration.
• RadioButtonHandler (92%) — CornerRadius, StrokeColor,
  StrokeThickness. Compose Material 3 RadioButton has no border
  or corner slot at all — needs a custom radio composable.
• ScrollViewHandler (94%) — HorizontalScrollBarVisibility,
  VerticalScrollBarVisibility. Modifier.verticalScroll doesn't
  expose scrollbar visibility (Android shows them implicitly on touch).
• ImageHandler / ImageButtonHandler (97%) — IsAnimationPlaying.
  Compose has no built-in animated image; needs coil-compose or
  similar.
• RefreshViewHandler (97%) — RefreshColor. Theming the spinner
  needs PullToRefreshDefaults.Indicator slot override.
• SliderHandler (97%) — ThumbImageSource. Compose Slider has
  no thumb-image slot short of writing a custom thumb composable.
• LabelHandler / PickerHandler / EntryHandler / EditorHandler
  / SearchBarHandler (each 98%) — VerticalTextAlignment. Compose's
  Box facade in this repo doesn't yet expose the contentAlignment
  slot (always passes null), so we can't wrap the text-field /
  label and align it. The mapper key is intentionally absent so it
  shows up in the gap analysis until the facade gains the slot.

Verification

• dotnet build src/Microsoft.AndroidX.Compose.Maui — clean.
• dotnet run scripts/maui-coverage.cs — regenerated
  docs/maui-coverage.md; numbers above match the report.
• Spot-checked Entry text-field key behaviour by reading the
  RenderWithSelection interception path; _suppressMirror guards
  the bidirectional Compose ⇄ MAUI loop and the
  ApplySelection/MapText paths skip writes that wouldn't change
  Compose's actual TextRange, so the mirror is stable across
  recompositions.

Out of scope (deliberately not touched)

• New red ❌ handlers (FlyoutViewHandler, NavigationViewHandler,
  ShapeViewHandler, WebViewHandler, menus, App / Window) — those
  remain larger separate PRs.
• The scripts/maui-coverage.cs parser — no false-positive key
  categories surfaced.
• Anything under src/Microsoft.AndroidX.Compose/ (facade); only the
  MAUI backend handlers under src/Microsoft.AndroidX.Compose.Maui/
  changed.

[Copilot]

Pull request overview

This PR significantly improves property-mapper key coverage across the Compose-backed MAUI handlers, raising overall coverage from 67.8% to 72.1% (830 → 883 keys). No new handler registrations are added — the focus is exclusively on wiring missing mapper keys in the 24 existing handlers. The bulk of the work centers on the text-input handlers (Entry, Editor, SearchBar), which are refactored to use Compose's OutlinedTextField(MutableState<TextFieldValue>) overload via a custom TextFieldValueState subclass that enforces MaxLength and mirrors cursor/selection state bidirectionally with MAUI.

Changes:

• Wired ~53 new property-mapper keys across 15 handlers (CharacterSpacing, CursorPosition, SelectionLength, IsOpen, HideSingle, MaximumVisible, ClipsToBounds, PlaceholderColor, StrokeColor/Thickness, CornerRadius, Font, Padding, Content, Title, etc.), bringing multiple handlers to 100% coverage.
• Refactored Entry/Editor/SearchBar onto Compose's TextFieldValue-based OutlinedTextField overload with a TextFieldValueState : MutableState<TextFieldValue> subclass that intercepts writes for MaxLength enforcement and two-way MAUI synchronization of text, cursor, and selection.
• Added TODO comments documenting deliberate gaps (dashed strokes, scrollbar visibility, animated images, thumb images, vertical text alignment) so the coverage report flags them honestly rather than hiding behind no-op mappers.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
ButtonHandler.cs	7 new keys: CharacterSpacing, Font, CornerRadius, StrokeColor/Thickness, Padding, Source (image via ImageSourceLoader)
EntryHandler.cs	Refactored to TextFieldValueState; added CursorPosition, SelectionLength, MaxLength, ClearButton, ReturnType, PlaceholderColor, CharacterSpacing, AutoCorrect, HorizontalTextAlignment
EditorHandler.cs	Parallel refactor to TextFieldValueState; added CursorPosition, SelectionLength, CharacterSpacing, AutoCorrect, PlaceholderColor, HorizontalTextAlignment
SearchBarHandler.cs	Parallel refactor to TextFieldValueState; added SearchIconColor, CancelButtonColor, ReturnType, MaxLength, CursorPosition, SelectionLength, CharacterSpacing, AutoCorrect, PlaceholderColor, HorizontalTextAlignment
LabelHandler.cs	CharacterSpacing, LineHeight, TextDecorations, Padding (via version-counter live-read)
PickerHandler.cs	CharacterSpacing, HorizontalTextAlignment, IsOpen (two-way), Items alias
DatePickerHandler.cs	CharacterSpacing, IsOpen (two-way via SetOpen helper)
TimePickerHandler.cs	CharacterSpacing, IsOpen (two-way via SetOpen helper)
IndicatorViewHandler.cs	HideSingle, MaximumVisible
RadioButtonHandler.cs	CharacterSpacing
LayoutHandler.cs	ClipsToBounds via Modifier.Clip(RectangleShape)
ScrollViewHandler.cs	Content mapper + version-counter for child-tree swaps
RefreshViewHandler.cs	IsRefreshEnabled (gates onRefresh callback)
PageHandler.cs	Title (deliberate no-op for mapper parity)
BorderHandler.cs	Shape alias key → MapShape
ImageHandler.cs	TODO comment for IsAnimationPlaying
ImageButtonHandler.cs	TODO comment for IsAnimationPlaying
SliderHandler.cs	TODO comment for ThumbImageSource
docs/maui-coverage.md	Regenerated coverage report reflecting new numbers