Liquid Glass Backdrop

Liquid glass: source, surface, rim

A backdrop library splits the effect in two. A source modifier records a composable's rendering into a graphics layer; a draw modifier on the surface samples that recording, pushes it through an effect stack, and paints the result behind itself:

// adapted
Modifier.drawBackdrop(
    backdrop = backdrop,
    shape = { shape }, highlight = { highlight },
    effects = {
        vibrancy()
        colorControls(brightness = 0.05f, contrast = 1f, saturation = 1.5f)
        blur(radius)
        // refraction height below the inradius (minDimension / 2), so top and bottom refractions
        // never meet at the medial axis — that meeting is the dark seam on a wide pill
        lens(size.minDimension / 4f, size.minDimension / 2f, false)
    },
    // darken as the background brightens, or bright artwork washes the glass out to white
    drawRect(scrimColor.copy(alpha = darken)) },
)

Three moving parts, in order of trouble caused: the rim highlight, the source/surface relationship, and the effect stack — the stack being a list of filters you tune by eye.

Traps

The rim highlight's default style is directional, not an outline. This is why a wide pill looks like glass and a 48 dp circular button beside it looks like nothing — same modifier, same parameters. Read the default style's shader rather than guessing:

// annotated — the library's own shader body, re-indented and `=`-aligned; the comments are added
float2 grad     = gradSdRoundedRect(centeredCoord, halfSize, gradRadius);  // outward edge normal
float2 normal   = float2(cos(angle), sin(angle));                          // one light direction
float d         = dot(grad, normal);
float intensity = pow(abs(d), falloff);
return color * intensity;

Brightness at a point on the outline is the alignment between that point's edge normal and one fixed direction. On a stadium an entire straight edge shares one axis-aligned normal, so both long edges light to a constant at once — most of the outline, uniformly, which reads as a rim. On a circle the normal sweeps every direction, so intensity runs |cos(θ − angle)|: full at two opposing points, zero at the two 90° away, under 70% over half the circumference — nothing left on a 48 dp button.

Two independent levers fix it. Swap the style for the library's uniform one (a colour and a blend mode; no angle, no falloff, therefore no direction). Or widen the rim, keeping the directional style: width feeds strokeWidth = ceil(width.toPx()) * 2, so the 0.5 dp default is the thinnest stroke the library draws and Highlight(width = 1.dp) puts back enough pixels for the surviving arc. The audited tree ships the second at every site that reaches for a lever, keeping them matched to the pills beside them as a style swap would not. In the 2.0.x line resolved here the default is white at 0.5 alpha, additive, 45°, falloff 1; the uniform style white at 0.38 alpha, additive.

The backdrop source must be a sibling of the glass, never its parent. The source records what it draws and the surface reads that recording while drawing itself, so a surface inside the source makes the recording depend on its own output — the loop stops the shader in the draw pass:

// adapted — the source is the content column; the glass buttons are siblings placed with align()
Box(Modifier.fillMaxWidth()) {
    Column(Modifier.fillMaxWidth().layerBackdrop(headerBackdrop)) {
        Spacer(Modifier.height(48.dp))              // reserves the strip the buttons sit over
        …artwork and text…
    }
    GlassIconButton(headerBackdrop, …, modifier = Modifier.align(Alignment.TopStart))
}

A source that draws nothing gives the glass nothing to bend — and the cure is a ground, not a retreat. Over an empty region the output is the surface scrim and nothing else: on a flat page the control renders as a grey coin, worse than no chrome at all. That is a full arc here — glass introduced, then removed for a plain icon button with exactly that reasoning written into the commit, then restored once the page had a ground worth refracting. The ground is a matchParentSize() box carrying the page colour plus the page's ambient tone, and the source is that box — never the content column, which is mostly transparent and rendered the control as a solid dark coin over a tinted page. Where the content genuinely draws (a list whose first item is a full-width image header) it is the right source, its strip reserved by a Spacer. The glass must also float over the ground rather than ride the scrolling column, which scrolls it away on a flat page.

The default tint assumes a forced-dark screen. These controls are usually built first for screens inside a forced-dark subtree, where a white glyph always sits over dark artwork and Color.White is sound. On a theme-following page it is white-on-white at light theme — and the flat fallback an off-setting swaps in is light too, so the glyph vanishes in both materials. Pass the scheme's on-surface token instead; force-dark-immersive-subtree is why the donor screens never hit this.

Run the discriminating experiment before changing the code. When one glass widget works and another does not, swap their positions: if the effect follows the widget the cause is its own geometry (the rim trap above), if it follows the position it is what the source records there. Four rebuilds went on plausible fixes first — widening the button to 96 dp (a longer edge catches more of the sweep, so the rim does appear: a symptom, not a fix), shrinking the lens radii, moving the source between a full-size box and the content column, and painting artwork in at low alpha.

Keep the press gesture observe-only. The glass scales up and draws a pointer-following glow on press, and it wraps buttons that need their own clicks. The recogniser must never consume(), must accept already-consumed downs (awaitFirstDown(requireUnconsumed = false)), and must bail when a change has been consumed — or the surface animates while the button inside it stops responding.

Check whether the library ships a variant for your other targets before writing a no-op actual. Assuming a shader-heavy effect is one-platform and stubbing the desktop actual leaves it dead code there that nobody notices — it compiles and renders a plain rounded box. If the artifact is multiplatform the abstraction has nothing to abstract: delete the expect class, keep a typealias so call sites do not churn, and move the effect into common code — which the class blocked anyway.

Verifying it

1. Confirm the default rim is directional and the uniform one is not, out of the dependency cache, no build. The range starts at the uniform block, not at half4 main:

   JAR=$(find ~/.gradle/caches -name '*backdrop*.jar' | head -1)
   DEF=$(unzip -l "$JAR"   | awk '/HighlightStyle\$Default\.class/{print $4}')
   PLAIN=$(unzip -l "$JAR" | awk '/HighlightStyle\$Plain\.class/{print $4}')
   unzip -p "$JAR" "$DEF"   | strings -n 6 | sed -n '/uniform float2 size/,/return color/p'
   unzip -p "$JAR" "$PLAIN" | strings -n 6 | grep -c 'angle\|falloff'
   

   → observed: uniform float angle and uniform float falloff declared, then the dot(grad, normal) main quoted above; then 0 — the uniform style ships no shader source at all.
2. List every backdrop source; read each one's enclosing block, and what it draws. grep -rn 'layerBackdrop(' --include='*.kt' . | grep -v 'fun Modifier' → observed: every hit is a Box/Column sibling of the glass it feeds — including the analytics screen, whose source is the LazyColumn itself (first item an image header, so it draws), buttons placed after it by align(). A source whose subtree holds a drawBackdrop for the same backdrop is the crash; a bare transparent source is the grey coin.
3. See which rim lever each call site reaches for. grep -rn 'highlight = ' --include='*.kt' . | grep -v 'fun \|highlight: ' → observed: six sites pass Highlight(width = 1.dp) (the wider-rim lever, style left directional), one bar passes Highlight.Default.copy(alpha = …), nothing passes the uniform style. Two 48 dp back buttons on the theme-following pair are on the bare default — the first ones to look at.
4. Resize one glass button between circle and wide pill, everything else unchanged: a rim appearing only in the pill state is the highlight trap. If both levers rescue it, geometry is confirmed.

Related: compose-desktop-runtime-hardening (shader-adjacent crashes from the rendering backend rather than your draw code) and shell-background-is-not-scheme-background (what a ground is made of).