Background layer: persistent content beneath every screen

[shanerbaner82]

A chrome contributor + native root hosts that render arbitrary content (a map, a video, a canvas) beneath the screen content at a stable structural position — mounted once at the root, so it persists across tab switches and pushes instead of re-initializing per screen.

• Discovery mirrors the floating overlay: layouts opt in with HasBackgroundLayer / a backgroundLayer() builder; screens override via backgroundLayerOverride() or hide with hidesBackgroundLayer.
• The background_layer sentinel element travels in the published tree; iOS (NativeUIBackgroundLayerHost.swift) and Android (NativeUIBackgroundLayerHost.kt) hosts registered on core's NativeRootHostRegistry pull it out and compose it under the content (tabs-hosted variant handled on Android).

Draft — in-flight feature branch packaged from the working tree so it has a home; review/finish at leisure.

[shanerbaner82]

Pushed fixes from review: the Android host now compiles (dropped the hostedByChrome argument that NativeUIBackgroundLayerHost never declared — the chrome hand-off can return alongside the core tabs work), the leftover bisect experiment is gone so the layer renders beneath content again, InteractsWithBackgroundLayer ships mirroring the overlay/drawer traits, the floating-overlay docblock is back on its method, and BackgroundLayerTest covers the builder, sentinel, and all three opt-out spellings (suite: 207 passing).

Still blocked before undrafting: Android touch routing under tabs needs the core-side change (hosting the layer inside the Scaffold body) that currently lives uncommitted in the core working tree, and iOS tabs have no transparency coordination yet (background_layer is unknown to core's iOS resources — an opaque TabView screen hides the layer).

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[shanerbaner82]

Field note from building a demo against this branch: the iOS transparency gap applies to NavigationStack chrome too, not just tabs — core folds root hosts around the whole tree outside the nav container (SwiftUINodeRenderer.swift:51 → NativeRootHostRegistry.wrap), so any screen with native chrome paints the system background over the layer and it never shows. A chrome-less screen (layout with usesNativeChrome: false, no nav bar) displays the layer correctly. Worth either documenting chrome-less as the supported iOS shape for now, or coordinating transparency from the stack/tab renderers when a background_layer sentinel is present (the Android LocalBackgroundLayerPresent pattern).

🤖 Generated with Claude Code