An ambient tone layer behind flat pages

Image-backed screens get their character from the image; settings, notifications and other flat pages get nothing and end up looking like a different app. One shared layer fixes that: an angled two-stop gradient from a tone into the page colour, capped at a fixed height.

// adapted
val AmbientGlowHeight = 360.dp
@Composable
fun AmbientThemeGlow(modifier: Modifier = Modifier, tint: Color? = null) {
    val pageBackground = /* what is ACTUALLY painted behind this page */
    val glow by animateColorAsState(tween(500), targetValue = when {
        tint == null -> pageBackground                    // collapses → layer is invisible
        isLightTheme -> lerp(tint, Color.White, 0.85f)    // pastel lift
        else         -> tint.rgbFactor(0.45f)             // deepen
    })
    Box(modifier.fillMaxWidth().height(AmbientGlowHeight)
            .angledGradientBackground(listOf(glow, pageBackground), 25f)) {
        Box(Modifier.fillMaxWidth().height(180.dp).align(Alignment.BottomCenter)
            .background(artworkScrimBrush(pageBackground)))   // eases the tail — no edge
    }
}

Call it as the first statement of the screen body, before the list — no wrapper:

// adapted — inside composable<SettingsDestination> { SettingScreen(…) }
AmbientThemeGlow(tint = rememberGlowTint(currentRecord?.imageUrl))
LazyColumn(state = listState, …) { … }

Traps

No wrapper is needed, and adding one costs you. A navigation destination's content already composes into a stacking container — NavHost takes a contentAlignment, which only means anything if children draw on top of one another — so an earlier sibling is the layer underneath. Wrapping the screen in your own Box instead re-parents it, changes every child's constraints, and is the usual reason a scroll container measures differently after a "purely visual" change.

A null tone must collapse the gradient, not substitute a colour. With nothing selected the layer should be invisible: make both gradient stops the page colour and it still draws, it just draws nothing you can see. Substituting the theme's primary as a "reasonable fallback" means an idle app glows for a record that is not playing. One level down, Color.Unspecified is the sentinel for "not resolved yet", never a real colour, or the first frames flash the wrong tone (unknown-not-a-valid-score).

The tail must aim at what is actually painted behind the page. On a shell that wraps content in a panel colour, a tail aimed at colorScheme.background ends on a hard seam exactly where the layer stops — shell-background-is-not-scheme-background in full, and why this resolves a page background rather than reading the scheme.

Animate the tone, so it breathes rather than pops. It arrives asynchronously (an image decode, a palette pass) and a hard swap flashes across the top third of the page; animating also makes the null case free, since collapsing to the page colour becomes a fade out.

The mixing factor belongs with the source of the tone, not with the layer. A mid-saturated image colour and a pastel theme colour need different treatment — the multiply that darkens the first attractively renders the second near-black. The shared layer deepens by 0.45 while the screen feeding its own image-derived tone uses 0.30; wanting one number means a caller is about to look wrong.

The layer must scroll away with the list, and the offset must be read in the draw phase.

// adapted
AmbientThemeGlow(tint = …, modifier = Modifier.graphicsLayer {
    translationY =
        if (listState.firstVisibleItemIndex == 0) -listState.firstVisibleItemScrollOffset.toFloat()
        else -size.height          // parked off-screen once item 0 is gone
})

A graphicsLayer lambda runs at draw time, so scrolling redraws without recomposing; reading the same scroll state in the composable body recomposes the layer — and every sibling — per scrolled pixel.

The first item has to be taller than the layer, or the hand-off jumps. The branch above swaps from exact tracking to parked when item 0 leaves; if item 0 is shorter, the layer is still partly visible then and teleports. One screen here folded two list items into one so item 0 would clear the layer's height — a layout constraint the layer imposes on its host, invisible until someone splits that item up again.

A top app bar's default container colour covers the only part that carries colour. The tone is strongest at the very top, exactly where the bar sits: transparent container, and let the bar frost the glow rather than hide it.

The height is a shared constant because the hand-rolled copy sizes itself from it. That screen imports the height and nothing else, so hardcoding 360 there is how the two drift on the next design pass. (Its own doc claims a caller gates a frosting bar on it; none does — all three use pixel 0.)

A hand-rolled copy re-implements the collapse, and usually not exactly. The screen keeping its own version gets "invisible until a tone arrives" by defaulting its colour state, not by a null branch — but it defaults to the scheme background while its own tail aims at the resolved page background. Equal without a shell; with one, 360 dp of gradient is fully visible before any tone arrives — shell-background-is-not-scheme-background, showing up inside this skill's own example.

Screens that need more are right to keep their own copy. The shared layer takes one tone; a screen animating a tone out of its own data, or one that must sit inside its own blur source so the bar frosts it, keeps a hand-rolled copy — share the recipe and the height constant, not the composable.

Verifying it

# 1. Every user of the layer, plus everything importing its height constant — the hand-rolled
#    copies sit in that second group, and are what to re-check when the recipe changes.
grep -rn --include='*.kt' -e 'AmbientThemeGlow(' -e 'AmbientGlowHeight' . | grep -v '/build/'

→ observed: two screens call the shared composable; one imports only the height constant.

# 2. The draw-phase scroll-away — a layer without one hangs off the ceiling as content moves.
grep -rn -A4 --include='*.kt' 'graphicsLayer {' . | grep -v '/build/' | grep 'firstVisibleItemScrollOffset'

# 3. The null-collapse. The first branch must resolve to the page colour, not to a theme colour.
grep -rn -A4 --include='*.kt' 'tint == null ->' . | grep -v '/build/'

→ observed: exactly three scroll-away layers, one per ambient screen, each negating item 0's offset; and tint == null -> pageBackground, with the light and dark shaping branches after it.

# 4. The stacking claim, against the resolved library: the host's signature carries an alignment,
#    which only exists for a container that stacks its children.
JAR=$(find ~/.gradle/caches/modules-2 -name 'navigation-compose*.jar' | sort -V | tail -1) && echo "$JAR"
D=$(mktemp -d) && unzip -oq "$JAR" -d "$D" && javap -p -classpath "$D" androidx.navigation.compose.NavHostKt | grep -c 'ui\.Alignment'

→ observed: 7 overloads take an androidx.compose.ui.Alignment, one exposing its content lambda as a BoxScope. sort -V picks the newest cached artifact — read the echoed path.

5. By eye: with nothing selected, the page must be indistinguishable from the same page with the layer deleted. Then select something with a strong image, scroll to the bottom and back, and watch the moment item 0 leaves the viewport for a jump.