MAUI Phase 5 Slice 1: fix AndroidView fallback sizing for self-drawing views

[jonathanpeppers]

On-device verification turned up a real bug, fixed in this slice.

TL;DR

The original commit on this branch claimed every Phase 5 control worked unchanged through ComposeWalker's AndroidView { factory = view.ToPlatform(MauiContext) } fallback — without any device verification. On a Pixel 5, Shapes and GraphicsView paint nothing: their stock Android views (MauiShapeView, PlatformGraphicsView) collapse to 0 × 0 and their Drawable never draws. (Forcing platformView.SetBackgroundColor(Color.Magenta) was invisible — proved zero size, not zero paint.)

Root cause: the fallback never applied a Compose Modifier.Size derived from MAUI's WidthRequest / HeightRequest. Compose handed AndroidView an unbounded slot. Compose-native handlers like BoxViewHandler already did this themselves; the fallback was the only path missing it.

Fix is one switch expression in ComposeWalker.Render. No new handlers, no new public surface.

Per-control verdict (on-device, Pixel 5)

Control	Verdict
Shapes (Rectangle / RoundRectangle / Ellipse / Line / Polygon / Polyline / Path)	Works via fallback after the fix. All 7 shape kinds + fill-width Rectangle render.
GraphicsView (IDrawable)	Works via fallback after the fix. Invalidate() repaints — tap-to-reshuffle bumps the seed and the canvas redraws.
WebView	Works via fallback (handler shape implies parity; sample page deleted — added no Compose-specific value).
HybridWebView	Deferred. Same handler shape as WebView, expected to work — not part of this slice's verification matrix.
BlazorWebView	Works via fallback by construction (ViewHandler over Android WebView); not exercised to avoid the package dependency.
Grid / AbsoluteLayout / FlexLayout / StackLayout	Works via fallback. Already shipping unmodified on HomePage.xaml's Grid chrome since Phase 1. Sample page deleted.

Sample page changes

Kept (on-device reproducers for the bug):

• Pages/ShapesPage.xaml(.cs)
• Pages/GraphicsViewPage.xaml(.cs)

Deleted (the controls work via the fallback after the fix and the demos surfaced no Compose-specific behaviour worth a gallery slot):

• Pages/WebViewPage.xaml(.cs)
• Pages/HybridWebViewPage.xaml(.cs) + Resources/Raw/hybridroot/index.html
• Pages/LayoutsPage.xaml(.cs)

Out of scope

• Compose-native shape handlers via Canvas + DrawScope — parked behind Bind drawing primitives: Canvas, Modifier.drawBehind/drawWithContent, DrawScope, Brush, Path, Shape factories #64.
• Compose-native GraphicsView via DrawScope shim — parked behind Bind drawing primitives: Canvas, Modifier.drawBehind/drawWithContent, DrawScope, Brush, Path, Shape factories #64.
• Custom Layout {} adapter (closes per-leaf ComposeView islands inside fallback-hosted layouts) — parked behind Bind custom Layout {} primitive — Measurable / Placeable / MeasureScope #144.
• HybridWebView on-device spot-check — deferred; expected parity.

Verification

• dotnet test src/Microsoft.AndroidX.Compose.SourceGenerators.Tests = 155 passed.
• dotnet build src/Microsoft.AndroidX.Compose clean.
• dotnet build src/Microsoft.AndroidX.Compose.Maui clean.
• dotnet build src/Microsoft.AndroidX.Compose.Maui.Sample clean (zero warnings, zero errors).
• dotnet build src/Microsoft.AndroidX.Compose.Maui.Sample -t:Install clean.
• dotnet build src/Microsoft.AndroidX.Compose.Gallery clean.
• On-device on a Pixel 5: deployed, exercised both kept sample pages — Shapes and GraphicsView render and respond as designed.

[Copilot]

Pull request overview

This PR closes Phase 5 Slice 1 by adding investigation-only sample pages that verify various MAUI controls (Shapes, GraphicsView, WebView, HybridWebView, and unsupported layouts) render correctly through the existing AndroidView fallback path in ComposeWalker. No new Compose-backed handlers or library code changes are introduced — the PR demonstrates that the stock MAUI handlers work as-is when hosted inside a Compose composition via AndroidView interop.

Changes:

• Five new sample pages (ShapesPage, GraphicsViewPage, WebViewPage, HybridWebViewPage, LayoutsPage) with XAML + code-behind exercising every Phase 5 control through unmodified stock handlers.
• Shell route registrations and HomePage catalog entries wiring the new pages into the sample app, plus a hybridroot/index.html raw asset for the HybridWebView JS bridge round-trip demo.
• docs/maui-backend.md updated with a comprehensive Phase 5 Slice 1 subsection: per-control verdict matrix, verification gap disclosure, intentional exclusions, and follow-up tracking.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Pages/ShapesPage.xaml(.cs)	New XAML page exercising all seven Shapes subclasses via the stock ShapeViewHandler fallback.
Pages/GraphicsViewPage.xaml(.cs)	New page with a synthetic IDrawable sparkline + tap-to-reshuffle, verifying GraphicsViewHandler fallback + Invalidate().
Pages/WebViewPage.xaml(.cs)	New page with HtmlWebViewSource inline JS counter + about:blank URL baseline.
Pages/HybridWebViewPage.xaml(.cs)	New page verifying HybridWebView JS bridge round-trip (SendRawMessage/RawMessageReceived).
Pages/LayoutsPage.xaml(.cs)	New page hosting Grid, AbsoluteLayout, FlexLayout via the AndroidView fallback.
Resources/Raw/hybridroot/index.html	Minimal HTML + JS for the HybridWebView bridge demo.
AppShell.xaml.cs	Shell route registrations for the five new pages.
HomePage.xaml.cs	Catalog entries for the five new demos.
docs/maui-backend.md	Replaces brief Phase 5 stub with full investigation report, per-control matrix, and follow-up plan.