husi-topbar

name	husi-topbar
description	Husi project's topbar design system. Use whenever adding a new screen in composeApp/, editing a Scaffold, building Compose UI with a topBar / tabs / search bar, or wiring scroll-driven color changes (scrollBehavior / pinnedScrollBehavior). If a topbar is involved at all, consult this skill — do not reach for Material 3's TopAppBar or AppBarWithSearch directly.

Husi Topbar Design

Husi replaces Material 3's default TopAppBar / AppBarWithSearch with a set of in-house "capsule" components. All of them live in composeApp/src/commonMain/kotlin/fr/husi/compose/CapsuleTopBar.kt.

Before writing any topbar, answer one question first: does the topbar sit directly above the scrollable content, or is there something else (tabs, search, etc.) attached underneath? That choice determines which pattern to use.

Component cheat sheet

Component	Purpose
CapsuleTopBar	Replacement for TopAppBar: nav icon + title + actions. Title auto-marquees with edge feathering when it overflows — see "Long titles".
CapsuleSearchTopBar	Replacement for AppBarWithSearch: nav icon + search capsule + actions
CapsuleSearchInputField	Input field wrapper for use inside CapsuleSearchTopBar. Handles centered placeholder + faded leading icon in the collapsed state
CapsuleActionButton	Wraps an action icon (e.g. SimpleIconButton) in capsule styling. Every entry inside actions = { ... } should be wrapped in one of these
PlatformMenuIcon	Cross-platform helper for navigationIcon: returns a menu IconButton on Android, returns null on desktop (desktop has no drawer)

Visual contract: the capsule fills are semi-transparent (surfaceContainer.copy(alpha = 0.75f))with a 1dp outlineVariant border. The topbar itself has no background — it floats over whatever is beneath. The visible "background" of the topbar comes from either an outer Surface (Pattern B) or the scrolling content showing through (Pattern A).

Long titles

CapsuleTopBar handles title overflow automatically. The title slot is wrapped in Box(Modifier.weight(1f)) so the pill is bounded to the row space left over after the nav icon and actions, and inside the pill the slot content is wrapped in Modifier.basicMarquee(). When the content's intrinsic width exceeds the pill's bounded width, a 16dp horizontal edge feather is added on each side via the shared Modifier.fadingEdge(...) from composeApp/src/commonMain/kotlin/fr/husi/compose/Fading.kt. Short titles render at intrinsic width with no marquee and no fade; actions stay right-pinned in both cases.

The title pill itself provides a bounded ripple via the internal PillCapsule surface, even though the click has no business action. Keep that empty click handler in the component so all topbar titles give consistent press feedback, and do not wrap caller-provided title content in another clickable/ripple modifier.

Caller-side rule: just pass title = { Text(stringResource(...)) }. Do not add maxLines = 1, softWrap = false, your own Modifier.basicMarquee(), or any width constraint on the title — the component already does all of that, and stacking marquees / constraints breaks the layout.

Modifier.fadingEdge(...) is reusable outside the topbar. Pass a ScrollableState for scroll-driven fades (e.g. LazyColumn top/bottom — this is the common case, and the function defaults fadeStart = false, fadeEnd = true for that scenario), or omit the ScrollableState (it defaults to null) for unconditional fade like the topbar's marquee branch uses.

Pattern A — topbar sits directly above scrolling content

The common case. RouteScreen, GroupScreen, LogcatScreen, ConfigEditScreen, ConnectionDetail, AssetsScreen all use this.

val scrollBehavior = TopAppBarDefaults.pinnedScrollBehavior()
val windowInsets = WindowInsets.safeDrawing

Scaffold(
    modifier = modifier
        .fillMaxSize()
        .nestedScroll(scrollBehavior.nestedScrollConnection),
    topBar = {
        CapsuleTopBar(
            navigationIcon = PlatformMenuIcon(
                imageVector = vectorResource(Res.drawable.menu),
                contentDescription = stringResource(Res.string.menu),
                onClick = onDrawerClick,
            ),
            title = { Text(stringResource(Res.string.menu_xxx)) },
            actions = {
                CapsuleActionButton {
                    SimpleIconButton(
                        imageVector = vectorResource(Res.drawable.update),
                        contentDescription = stringResource(Res.string.update),
                        onClick = { /* ... */ },
                    )
                }
                // additional actions, each wrapped in its own CapsuleActionButton
            },
            windowInsets = windowInsets.only(WindowInsetsSides.Top + WindowInsetsSides.Horizontal),
            scrollBehavior = scrollBehavior,
        )
    },
) { innerPadding ->
    LazyColumn(
        contentPadding = innerPadding.withNavigation(),
        // ...
    ) { /* items */ }
}

Key points:

• The topbar does not need an outer Surface(color = ...). The capsules float on top of the LazyColumn and list items scrolling behind them is the intended look.
• Pass scrollBehavior to CapsuleTopBar even if you aren't reading overlappedFraction here. The component sets state.heightOffsetLimit via SideEffect; without that step, Pattern B can't read a meaningful overlappedFraction later if you ever upgrade the screen.
• Use innerPadding.withNavigation() for LazyColumn's contentPadding; use Modifier.paddingExceptBottom(innerPadding) for the Modifier form. Both ensure the bottom navigation/FAB area is preserved.
• Pass windowInsets.only(WindowInsetsSides.Top + WindowInsetsSides.Horizontal) so the bottom inset is left to the Scaffold (used by FAB / StatsBar).

Pattern B — topbar has tabs / search bar / other non-content attached below

Used by Configuration, Dashboard, Tools, AbstractAppList. The topbar + the attached element together form a single "header block", and the whole block should change color as the user scrolls ( smoothly lerping from containerColor to scrolledContainerColor).

val scrollBehavior = TopAppBarDefaults.pinnedScrollBehavior()
val topAppBarColors = TopAppBarDefaults.topAppBarColors()
val appBarContainerColor by animateColorAsState(
    targetValue = lerp(
        topAppBarColors.containerColor,
        topAppBarColors.scrolledContainerColor,
        scrollBehavior.state.overlappedFraction.fastCoerceIn(0f, 1f),
    ),
    animationSpec = spring(stiffness = Spring.StiffnessMediumLow),
    label = "appBarContainerColor",
)

Scaffold(
    modifier = modifier
        .fillMaxSize()
        .nestedScroll(scrollBehavior.nestedScrollConnection),
    topBar = {
        Surface(color = appBarContainerColor) {
            Column {
                CapsuleTopBar(
                    // ... navigationIcon / title / actions ...
                    windowInsets = windowInsets.only(WindowInsetsSides.Top + WindowInsetsSides.Horizontal),
                    scrollBehavior = scrollBehavior,
                )
                PrimaryTabRow(
                    selectedTabIndex = pagerState.currentPage,
                    containerColor = appBarContainerColor, // required
                ) { /* tabs */ }
            }
        }
    },
) { /* ... */ }

Three non-negotiable pieces, all required:

1. Surface(color = appBarContainerColor) wraps the whole header block (topbar + tabs/search/etc). CapsuleTopBar is transparent — the visible "background" is this Surface showing through.
2. scrollBehavior is passed into CapsuleTopBar / CapsuleSearchTopBar. Their internal SetHeightOffsetLimit sets state.heightOffsetLimit = -60.dp.toPx(). Without it, overlappedFraction's if (heightOffsetLimit != 0f) { ... } else { 0f } always returns 0 and the color never animates.
3. TabRow is given an explicit containerColor = appBarContainerColor. PrimaryTabRow / PrimaryScrollableTabRow default to an opaque surface color that masks the outer Surface's animated color.

Search topbar variant

Dashboard shows CapsuleSearchTopBar on the Connections page and CapsuleTopBar elsewhere, sharing the same scrollBehavior:

Surface(color = appBarContainerColor) {
    Column {
        if (isConnectionsPage) {
            CapsuleSearchTopBar(
                inputField = searchInputField,
                navigationIcon = PlatformMenuIcon(...
            ),
            actions = { /* CapsuleActionButton { ... } */ },
            windowInsets = windowInsets.only(WindowInsetsSides.Top + WindowInsetsSides.Horizontal),
            scrollBehavior = scrollBehavior,
            )
        } else {
            CapsuleTopBar(
                navigationIcon = PlatformMenuIcon(...
            ),
            title = { Text(stringResource(Res.string.menu_dashboard)) },
            windowInsets = windowInsets.only(WindowInsetsSides.Top + WindowInsetsSides.Horizontal),
            scrollBehavior = scrollBehavior,
            )
        }
        PrimaryTabRow(
            selectedTabIndex = pagerState.currentPage,
            containerColor = appBarContainerColor,
        ) { /* ... */ }
    }
}

CapsuleSearchInputField is the standard way to construct inputField:

val searchInputField: @Composable () -> Unit = {
    CapsuleSearchInputField(
        textFieldState = vm.searchTextFieldState,
        searchBarState = searchBarState,
        onSearch = { focusManager.clearFocus() },
        placeholder = { Text(stringResource(Res.string.search_go)) },
        leadingIcon = { Icon(vectorResource(Res.drawable.search), null) },
        trailingIcon = if (searchBarState.currentValue == SearchBarValue.Expanded) {
            { SimpleIconButton(/* close icon */) }
        } else null,
    )
}

Search capsule gestures:

• CapsuleSearchTopBar supports onSearchPillLongPress for a secondary action on the search capsule shell. Use it when migrating an old title-bar long-press behavior, such as Configuration's "jump to selected proxy" action.
• Do not repurpose the search capsule's normal tap for navigation or scrolling. A tap belongs to the search field and should keep expanding/focusing search.
• If a screen needs a "tap the current section again to jump within the list" behavior, attach it to the selected tab's onClick branch instead of the search field.

Material 3 SearchBar special case (AbstractAppList)

AbstractAppList (the per-app selection screen) places Material 3's SearchBar under the topbar — not CapsuleSearchTopBar, because this screen needs ExpandedFullScreenSearchBar integration. SearchBar has its own pill styling driven by SearchBarTokens.ContainerColor, and its perceived " border" comes from the color contrast against whatever surrounds it.

Do not pass colors = SearchBarDefaults.colors(containerColor = appBarContainerColor). Matching the surrounding color makes the pill blend into the background and the border disappears. Leave SearchBar's colors alone and let the outer Surface animate around it (the topbar area, the gutters next to the SearchBar, and extraTopBarContent):

Surface(color = appBarContainerColor) {
    Column {
        CapsuleTopBar(..., scrollBehavior = scrollBehavior)
        SearchBar(
            state = searchBarState,
            inputField = searchInputField,
            modifier = Modifier.fillMaxWidth(),
            // no `colors =` — keep SearchBar's own pill look
        )
        extraTopBarContent()
    }
}

Pattern selection

What does your topBar look like?
├── A single bar sitting directly above a LazyColumn / Column of scrolling content
│   → Pattern A: CapsuleTopBar with no Surface wrapper
│
└── Topbar with tabs / search / other non-content attached underneath
    └── The whole block must change color as content scrolls behind it
        → Pattern B:
        ├── Compute appBarContainerColor with animateColorAsState + lerp
        ├── Wrap header in Surface(color = appBarContainerColor)
        ├── Pass scrollBehavior into CapsuleTopBar / CapsuleSearchTopBar
        └── Pass containerColor = appBarContainerColor to the TabRow
            (Material 3 SearchBar is the exception — keep its default colors)

Common pitfalls

• overlappedFraction is always 0 / color never changes. You forgot to pass scrollBehavior into CapsuleTopBar / CapsuleSearchTopBar. Their internal SetHeightOffsetLimit is the prerequisite for the color animation. Material 3's TopAppBar sets heightOffsetLimit = -placeable.height inside its Layout; the capsule components achieve the same via SideEffect + a fixed 60.dp, but only when scrollBehavior is actually wired in.
• Tabs stay one color while only the topbar half animates. PrimaryTabRow defaults to an opaque surface containerColor that hides the outer Surface. Explicitly pass containerColor = appBarContainerColor.
• SearchBar lost its border / the pill is invisible. Don't override SearchBar's colors.containerColor. Its outline is the color contrast with the surrounding area.
• Action icons look wrong / don't have the capsule background. Every SimpleIconButton inside actions = { ... } must be wrapped in CapsuleActionButton { ... }. Don't drop bare icon buttons in directly.
• Empty slot where the nav icon should be on desktop. Use PlatformMenuIcon(...) for the navigation icon. It returns null on desktop, and the topbar's Row collapses that slot automatically. A hand-rolled SimpleIconButton will produce a desktop menu button that shouldn't exist.
• Bottom inset fighting with FAB / StatsBar. Give the topbar windowInsets.only(WindowInsetsSides.Top + WindowInsetsSides.Horizontal) so the Scaffold owns the bottom inset. Use innerPadding.withNavigation() on LazyColumn contentPadding, or Modifier.paddingExceptBottom(innerPadding) for the Modifier form.
• Manually adding Modifier.basicMarquee(), maxLines = 1, or width constraints to a title Text. CapsuleTopBar already bounds the title pill (Box(Modifier.weight(1f))) and applies basicMarquee + a conditional fadingEdge based on overflow detection inside the pill. Adding your own marquee compounds the animation, and width constraints fight the slot's weighted layout. Just pass a plain Text(stringResource(...)).

Reference implementations

• Pattern A: composeApp/src/commonMain/kotlin/fr/husi/ui/RouteScreen.kt, GroupScreen.kt, LogcatScreen.kt
• Pattern B (tabs): composeApp/src/commonMain/kotlin/fr/husi/ui/tools/ToolsScreen.kt
• Pattern B (tabs + conditional search): composeApp/src/commonMain/kotlin/fr/husi/ui/dashboard/Dashboard.kt
• Pattern B (search + tabs): composeApp/src/commonMain/kotlin/fr/husi/ui/configuration/ConfigurationScreen.kt
• Pattern B (Material 3 SearchBar special case): composeApp/src/androidMain/kotlin/fr/husi/ui/AbstractAppList.kt
• Component source: composeApp/src/commonMain/kotlin/fr/husi/compose/CapsuleTopBar.kt