name: compose-expert description: Shared Compose UI in commonsUI/: screens, components, the nav shell (ScreenLayoutSpec, AppShellLayout, Navigation 3 back stacks), component catalog, RichTextParser, Compose API/perf review. Use when writing or reviewing shared UI.

compose-expert

Where shared Compose UI lives in Amethyst and the house rules for it. What goes in commons vs commonsUI vs a platform shim is in AGENTS.md ("Sharing Philosophy", "Feature Workflow") and commonsUI/ARCHITECTURE.md; this skill does not repeat it.

Delegate: Activity / manifest / flavors -> android-expert; Window, tray, menu bar -> desktop-expert; source-set placement, expect/actual, extracting an Android composable -> kotlin-multiplatform; feeds and Account state -> account-state; Flow and coroutine scopes -> kotlin-coroutines.

Where things live

All paths below are under commonsUI/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/.

The Android app's AmethystTheme wrapper is still in amethyst/src/main/java/com/vitorpamplona/amethyst/ui/theme/Theme.kt; the palettes it applies are shared.

The shell and window tiers

One shell for every window, chosen by size, not by platform (AGENTS.md "Navigation Shell"):

• AmethystApp (ui/app/AmethystApp.kt) is the root both front ends call. Android's MainActivity passes AndroidAppRoot; the desktop window (AmethystDesktop.kt) passes a DesktopAppRoot. AppRoot (ui/app/AppRoot.kt) is the port each platform implements: createAccountViewModel, rememberScreenLayoutSpec, registerDestinations, ShellOverlay, NavigationEffects, ...
• ScreenLayoutSpec.navigationStyle is BOTTOM_BAR, NAV_RAIL or PERMANENT_DRAWER; hasRoomForNotificationPanel docks the notification panel. isLargeScreen is "not the bottom bar". Read it with LocalScreenLayout.current, never with a platform check.
• AppShellLayout (ui/navigation/shell/) lays out the bar / rail / drawer; ui/navigation/bottombars/ holds AppBottomBar and AppNavigationRail; ui/navigation/deck/DeckArea.kt adds the opt-in deck columns on windows of 1,200 dp+.

Navigation 3

The app owns its back stack as snapshot state; NavDisplay only renders it. No NavController, no graph. Full rules in references/navigation.md.

• Routes: @Serializable sealed class Route in commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/navigation/Routes.kt. The back stack is saved through kotlinx-serialization, so every route argument must be serializable.
• NavBackStacks (same package) holds stack (never empty, starts at Route.Home) and savedTabs (the root entry of each bottom-bar tab you left, so its ViewModels and scroll survive). A NavStackEntry's contentKey is "nav-$id": saved state and ViewModels key on it, so the same route opened twice is two screens.
• Nav (ui/navigation/navs/Nav.kt) implements INav: nav, navDrawer, newStack, navBottomBar, popBack, popUpTo, each after the keyboard settles (ImeSettler).
• Register a screen once with a builder from ui/navigation/host/NavDestinations.kt. Shared screens go in NavDestinations.sharedDestinations (ui/navigation/host/SharedDestinations.kt); Android-only ones in androidDestinations in amethyst/src/main/java/com/vitorpamplona/amethyst/ui/navigation/AppNavigation.kt. The builder picks the motion and the reading-column cap:

// from SharedDestinations.kt
composableCapped<Route.Home> { HomeScreen(accountViewModel, nav) }
composable<Route.Message> { MessagesScreen(accountViewModel, nav) }
composableFromEnd<Route.Polls> { PollsScreen(accountViewModel, nav) }
composableFromEnd<Route.Browser>(capWidth = false) { BrowserScreen(accountViewModel, nav) }
composableFromBottomArgs<Route.AwardBadge> { AwardBadgeScreen(it.kind, it.pubKeyHex, it.dTag, accountViewModel, nav) }

A route nobody registered fails the first time it is opened (the desktop installs an unavailable screen instead). Gate a menu item on canOpen(route) when a front end may lack it.

Composable conventions that differ from the default

• Strings come from Res, read with stringRes(Res.string.x); no literal user-visible text in a shared composable, no Android R.string. Default label parameters are StringResource (see AddButton in ui/components/ActionButtons.kt).
• UI-state collections are kotlinx.collections.immutable (ImmutableList, ImmutableMap, persistentListOf), an api dependency of commonsUI. The parser output (RichTextViewerState, ParagraphState) and the platform ports already use them; pass those, not List, into composables.
• @Immutable / @Stable mark the state classes and port interfaces handed to composables (ScreenLayoutSpec, AppPlatform, PlatformPermission). Only mark a class you can keep truly immutable.
• Theme-dependent values come from the color scheme: MaterialTheme.colorScheme.isLight (memoized in AmethystColorScheme.kt), not a hand-rolled luminance check.
• Icons follow AGENTS.md "Icons": icons/*.kt compile into amethyst_icons.ttf and draw through AmethystIconGlyph; Material icons are MaterialSymbols from a subset font, so a new codepoint needs tools/material-symbols-subset/subset.sh. Robohash is the one ImageVector-built asset: references/icon-assets.md.
• Parse rich text through CachedRichTextParser, never per recomposition: references/rich-text-parsing.md.
• Permissions in shared UI go through an expect fun rememberXPermission(): PlatformPermission (see ui/components/LocationPermission.kt; the Android actual wraps Accompanist).

Reviewing a component's public API

For slot parameters, modifier placement, and the other API-shape checks, use references/api-style.md.

Performance: measure first

Feed jank in this app has been measured as GC-bound and RenderThread-bound, with composition a small share of frame time. Stability annotations, derivedStateOf and deferred reads only help when a trace shows composition is the cost. Profile a release-like build (the benchmark build type in amethyst/build.gradle.kts is minified and profileable) before you apply them, and measure again afterwards.

Compose compiler reports are configured only for :amethyst (composeCompiler { reportsDestination ... } near line 551 of amethyst/build.gradle.kts, written to amethyst/build/compose_compiler/). They cover only composables compiled in that module. Shared composables are compiled in :commonsUI, which has no such block. To inspect their skippability, add the block to commonsUI/build.gradle.kts temporarily, and don't commit it.

See it running

A screen change is not done until you've looked at it: tools/desktop-run/run-headless.sh start runs the desktop app, which is this same UI, on a virtual display and screenshots it. AMETHYST_SCREEN=480x900x24 shows the phone tier. See tools/desktop-run/README.md and desktop-expert.

References

• references/navigation.md: back-stack operations, rendering, transitions, intents, tests
• references/shared-composables-catalog.md: the reusable components and their signatures
• references/rich-text-parsing.md: RichTextParser, segments, CachedRichTextParser, NIP-92 imeta
• references/icon-assets.md: robohash avatars
• references/api-style.md: component API review checklist (slots, modifiers)