Imported from j-roskopf/Phoebe (.agents/skills/compose-modifier-and-layout-style/SKILL.md). Install upstream with npx skills add j-roskopf/Phoebe --skill compose-modifier-and-layout-style. Copyright stays with the author.

Compose modifier and layout style

Core principle

A composable that emits layout is a leaf the parent places — the parent decides position, size, alignment, padding. The composable's job is structure (what's inside), not placement (where it goes). Three rules follow:

• Declare a modifier parameter and apply it to the root, so the parent can actually do its job. Hardcoding .fillMaxWidth() on a composable's root takes that decision away from every future caller.
• Construct modifier chains as one fluent expression, not stepwise reassignments. Both compile to the same thing, but the chain reads as intent in one pass.
• Conditional rendering belongs where the condition applies. A layout call whose only content is one if exists solely to hold the condition — push the if outside instead.

These travel together because the same composable usually triggers all three: you declare its parameters (rule 1), the caller constructs a chain to position it (rules 2), and the body has a conditional you might be tempted to wrap (rule 3).

When to use this skill

• You're writing a @Composable fun that calls a layout (Box, Column, Row, LazyColumn, Text, Image, Surface, Card, Layout { … }, anything from compose.foundation.layout or compose.material*) and its signature has no modifier parameter, or has one that isn't applied to the root, or has a hardcoded .fillMaxWidth()/.padding(...) on the root.
• You see var m = Modifier followed by m = m.padding(…), m = m.background(…), etc.
• A modifier = … argument has three or more chained calls on a single line.
• A composable's body is Layout { if (cond) Content() } — one conditional, nothing else.

1. Declare a modifier parameter

For composables that emit layout, prefer a modifier parameter after required parameters and before content/lambda parameters, with a default of Modifier. The name is exactly modifier — not mod, not m, not wrapperModifier.

// ❌ BAD — no modifier param; caller can't position, size, or constrain this
@Composable
fun HomeScreenHeader(title: String, subtitle: String) {
    Column(
        modifier = Modifier
            .fillMaxWidth()
            .padding(horizontal = 16.dp),
        verticalArrangement = Arrangement.spacedBy(4.dp),
    ) {
        Text(title, style = MaterialTheme.typography.headlineLarge)
        Text(subtitle, style = MaterialTheme.typography.bodyMedium)
    }
}

// ✅ GOOD — parent decides width and padding; the composable describes structure only
@Composable
fun HomeScreenHeader(
    title: String,
    subtitle: String,
    modifier: Modifier = Modifier,
) {
    Column(
        modifier = modifier,
        verticalArrangement = Arrangement.spacedBy(4.dp),
    ) {
        Text(title, style = MaterialTheme.typography.headlineLarge)
        Text(subtitle, style = MaterialTheme.typography.bodyMedium)
    }
}

The caller now writes HomeScreenHeader(title, subtitle, Modifier.fillMaxWidth().padding(horizontal = 16.dp)) once, at the home screen — the only place that knows the layout actually wants those.

2. Apply the caller's modifier to the root, and apply it first

When the root layout already takes other arguments (alignment, arrangement, padding that's intrinsic to the composable), the caller-provided modifier still goes on the root layout's modifier parameter — and the composable's local chain is appended after.

// ❌ BAD — modifier accepted but never applied
@Composable
fun Avatar(url: String, modifier: Modifier = Modifier) {
    Image(painter = rememberAsyncImagePainter(url), contentDescription = null)
}

// ❌ BAD — applied to a child, not the root; caller's size/position changes don't take
@Composable
fun Avatar(url: String, modifier: Modifier = Modifier) {
    Box {
        Image(
            painter = rememberAsyncImagePainter(url),
            contentDescription = null,
            modifier = modifier,
        )
    }
}

// ❌ BAD — caller's modifier ends up last, so the composable's own size wins
@Composable
fun Avatar(url: String, modifier: Modifier = Modifier) {
    Image(
        painter = rememberAsyncImagePainter(url),
        contentDescription = null,
        modifier = Modifier
            .clip(CircleShape)
            .size(48.dp)
            .then(modifier),
    )
}

// ✅ GOOD — caller's modifier first, then the composable's intrinsic chain
@Composable
fun Avatar(url: String, modifier: Modifier = Modifier) {
    Image(
        painter = rememberAsyncImagePainter(url),
        contentDescription = null,
        modifier = modifier
            .clip(CircleShape)
            .size(48.dp),
    )
}

Order matters: in a modifier chain, the earlier segment is the outer wrapper. The caller's modifier should be the outermost so caller-provided .size(...) or .padding(...) can override the composable's defaults rather than being overridden by them.

3. Don't hardcode layout decisions on the root

If the composable's root has .fillMaxWidth(), .padding(horizontal = 16.dp), .height(56.dp), etc., the caller can't not have them. Those are layout choices the parent should own.

// ❌ BAD — every caller now fills max width whether they want to or not
@Composable
fun PrimaryButton(text: String, onClick: () -> Unit, modifier: Modifier = Modifier) {
    Button(
        onClick = onClick,
        modifier = modifier.fillMaxWidth(),   // ← hardcoded
    ) { Text(text) }
}

// ✅ GOOD — caller adds .fillMaxWidth() if (and only if) they want it
@Composable
fun PrimaryButton(text: String, onClick: () -> Unit, modifier: Modifier = Modifier) {
    Button(onClick = onClick, modifier = modifier) { Text(text) }
}

The carve-out is for modifiers that are part of the identity of the composable — what makes an Avatar an avatar (the .clip(CircleShape) and a default .size(48.dp)), not where it sits on the screen. Test: can you imagine a caller wanting a version of this composable without that modifier? If yes, push it out. If no (an avatar without clip(CircleShape) isn't an avatar), keep it — but put it after the caller's modifier in the chain (see §2).

4. Construct modifier chains as one fluent expression

Recomposition re-runs the composable body — every modifier expression is re-evaluated. Reassigning var modifier = step-by-step looks plausible but breaks the visual flow, invites further mutation, and produces nothing a chain doesn't.

// ❌ BAD — visual flow broken into reassignments; `var` invites more mutation
@Composable
fun Demo() {
    var m = Modifier
    m = m.padding(16.dp)
    m = m.fillMaxSize()
    Box(m) { }
}

// ❌ ALSO BAD — same shape, dressed up with .then()
@Composable
fun Demo() {
    var m = Modifier
    m = m.padding(16.dp)
    m = m.then(Modifier.fillMaxSize())
    Box(m) { }
}

// ✅ GOOD
@Composable
fun Demo() {
    val m = Modifier
        .padding(16.dp)
        .fillMaxSize()
    Box(m) { }
}

val, not var: once the chain is built, nothing should re-bind it. The reassignment shape is what makes var look necessary; the chain shape doesn't need it.

Inline at the call site is fine for short chains

For one or two calls, build the modifier inline. The "extract to a val" rule only earns its keep when the chain is long enough to be worth naming, or when the same chain repeats.

// ✅ GOOD — short chain inline
Box(modifier = Modifier.fillMaxWidth()) { … }
Box(modifier = Modifier.padding(8.dp).background(Color.Red)) { … }

Conditional segments stay on the chain

A common reason to reach for var is "the modifier depends on a condition." It doesn't — splice the condition inline:

// ✅ GOOD — conditional inside the chain, still one expression
Box(
    modifier = Modifier
        .fillMaxWidth()
        .then(if (selected) Modifier.background(Color.Red) else Modifier),
)

Modifier (the empty modifier) is the identity element for .then — it lets you keep the chain shape when one branch contributes nothing.

5. Multiline formatting at the call site

When a modifier argument's chain has three or more calls, format multiline with one call per line. Indent the chain so the dotted calls align beneath the value.

// ❌ BAD — three+ calls on one line; hard to scan
Box(
    modifier = modifier.fillMaxSize().padding(16.dp).weight(1f),
)

// ✅ GOOD
Box(
    modifier = modifier
        .fillMaxSize()
        .padding(16.dp)
        .weight(1f),
)

One or two calls stay on a single line — the threshold is the call count, not the character count. If a single call has very long arguments, that's a different problem (extract a val, or shorten the arguments).

This applies only to a parameter named modifier. Other fluent-style arguments aren't covered here.

6. Hoist single conditionals out of the layout

When a layout's only content is one if, the layout exists solely to "hold" the conditional. Move the if outside — the layout will only exist when it has something to show.

// ❌ BAD — Column always emitted; only its inner content is conditional
@Composable
fun A() {
    Column {
        if (showHeader) {
            Text("Title")
            Text("Subtitle")
        }
    }
}

// ✅ GOOD — Column only exists when it has content
@Composable
fun A() {
    if (showHeader) {
        Column {
            Text("Title")
            Text("Subtitle")
        }
    }
}

The benefit isn't a performance win — the runtime handles both fine — it's that the second form reads as "header section, conditionally." The first reads as "always-on column that may or may not have content."

The carve-outs (and why)

• Layout carries visual semantics that aren't conditional. When the layout call passes modifier, contentAlignment, horizontalArrangement, or verticalAlignment, those arguments describe the container, not the content. Hoisting the conditional either loses those (the container collapses with the content) or duplicates them into both branches. Leave it.

  // ✅ KEEP AS-IS — modifier on the container is doing visible work
  @Composable
  fun A(modifier: Modifier = Modifier) {
      Box(modifier = modifier) {
          if (something) {
              Text("Bleh1")
              Text("Bleh2")
          }
      }
  }

• There are siblings to the if. The layout has other content; the if is just one piece. Hoisting either pulls the siblings out (changing the layout) or leaves a different shape behind. Leave it.

• if … else … with both branches contributing composables. Both branches do work; nothing to hoist; the layout is the shared container.

  // ✅ KEEP AS-IS — both branches contribute to the layout
  Box {
      if (something) Text("Hint") else innerTextField()
  }

Quick reference

When NOT to apply

• Composables that don't emit layout. A @Composable fun computeColor(): Color or a @Composable @ReadOnlyComposable accessor doesn't emit a layout node. No modifier parameter needed (and a @ReadOnlyComposable couldn't accept one — see compose-state-authoring).
• @Preview functions. Previews are throwaway entry points; the framework calls them with no caller. A modifier parameter would be unused dead weight.
• Test-only composables inside *Test sources whose only caller is composeTestRule.setContent { … }. Same reasoning as previews.
• Internal layout primitives that take a modifier as their first required parameter (very rare; framework-level). The rule is "first optional param"; some private utilities legitimately have modifier upfront as required.
• Modifier assembled imperatively from animation state. A modifier built by appending values from Animatable or other procedural sources may legitimately need intermediate variables. The chain isn't the goal; readability is. If the chain becomes a worse expression, write the imperative form.
• Slot APIs that store modifiers in a data class or builder (rare; usually framework-level code). The fluent-chain idea is about user-site construction.
• Test composables pinning specific recomposition shapes — usually fine either way; don't refactor test composables purely for style.

The declaration-side rules (§1–§3) should not be skipped merely because "this composable is internal", "only used in one place", "I'd rather not have the extra parameter on the signature", or "we know all the callers already". Those are exactly the rationalisations that produce composables that become single-use the day someone wants to call them twice.

Red flags during review

Related

• compose-slot-api-pattern — the other half of declaring a reusable composable's public API: take @Composable () -> Unit slots for variable content. A reusable component takes both a modifier parameter and slots — caller owns placement and what to place.