Bind custom Layout {} primitive (closes #144)

.github/copilot-instructions.md

@@ -506,6 +506,20 @@ so `[CallerFilePath]` + `[CallerLineNumber]` slot keys inside
   Kotlin ctor needs JNI (mangled because `selection: TextRange` is a
   `@JvmInline value class`); everything else (`Text`/`Selection`/`Copy(…)`)
   is exposed by the runtime binding. See issue #204.
+- `Layout` exposes a low-level measure-and-place primitive: ctor takes a
+  user `Func<MeasureScope, IReadOnlyList<Measurable>, Constraints, MeasureResult>`
+  delegate, the `MeasurePolicy` parameter is built once via a tiny Java
+  helper that returns a SAM lambda — `MeasurePolicy` is a Kotlin
+  `fun interface`, so `javac` resolves the single abstract member
+  (`measure-3p2s80s`, mangled because `Constraints` is `@JvmInline value class`)
+  by signature via `LambdaMetafactory` and the source never has to spell the
+  illegal `-` identifier. Default interface methods (the four
+  `IntrinsicMeasureScope.*Intrinsic*` helpers) are inherited correctly by the
+  synthesized class. The SAM instance + JCW lambda are cached via
+  `composer.Remember` so JNI identity stays stable across recompositions.
+  None of (custom user-delegate ctor, wrapper-typed params not in
+  `ComposeValueTypes`, JCW with mutable `Body`) fit any `[ComposeFacade]`
+  phase. See issue #144.
 
 Applying `[ComposeFacade]` to an unsupported bridge emits CN3002 (unsupported
 param), CN3003 (scope misuse), CN3005 (invalid callback type), CN3006 (slot

docs/api-coverage.md

@@ -1,6 +1,6 @@
 # Jetpack Compose ⇄ Microsoft.AndroidX.Compose API coverage
 
-Generated by `scripts/api-comparison.cs` on 2026-06-11 21:52 UTC.
+Generated by `scripts/api-comparison.cs` on 2026-06-12 22:41 UTC.
 
 Source of truth: AndroidX `-sources.jar` files for the Compose artifact 
 versions this repo references (see `Directory.Build.targets`). Re-run after 
@@ -9,25 +9,25 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 ## Summary
 
 - **Total Kotlin public symbols in scope**: 2215
-- **Covered by Microsoft.AndroidX.Compose (any kind)**: 362 (**16.3%**)
+- **Covered by Microsoft.AndroidX.Compose (any kind)**: 375 (**16.9%**)
 
-- **@Composable functions**: 159 / 287 (**55.4%**)
+- **@Composable functions**: 160 / 287 (**55.7%**)
 
 ### Per-module coverage
 
 | Module | Composables | All public |
 | --- | --- | --- |
 | `animation` | 8/17 (47%) | 16/56 (29%) |
 | `animation-core` | 0/25 (0%) | 0/161 (0%) |
-| `foundation` | 15/42 (36%) | 45/356 (13%) |
+| `foundation` | 15/42 (36%) | 48/356 (13%) |
 | `foundation-layout` | 7/11 (64%) | 43/128 (34%) |
 | `material3` | 109/133 (82%) | 168/316 (53%) |
 | `runtime` | 6/20 (30%) | 13/200 (7%) |
 | `runtime-saveable` | 1/3 (33%) | 1/13 (8%) |
-| `ui` | 10/26 (38%) | 44/596 (7%) |
+| `ui` | 11/26 (42%) | 51/596 (9%) |
 | `ui-graphics` | 0/0 (0%) | 10/187 (5%) |
-| `ui-text` | 0/0 (0%) | 13/129 (10%) |
-| `ui-unit` | 0/0 (0%) | 5/49 (10%) |
+| `ui-text` | 0/0 (0%) | 14/129 (11%) |
+| `ui-unit` | 0/0 (0%) | 7/49 (14%) |
 | `activity-compose` | 1/5 (20%) | 2/11 (18%) |
 | `navigation-compose` | 2/5 (40%) | 2/13 (15%) |
 
@@ -344,7 +344,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [x] `SelectionContainer`(2) → type match
 - [x] `VerticalPager`(16) → type match
 
-### Other top-level functions — 22/156 (14%)
+### Other top-level functions — 25/156 (16%)
 
 - [ ] `AbsoluteCutCornerShape`(5)
 - [ ] `AbsoluteRoundedCornerShape`(5)
@@ -398,11 +398,11 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `CornerSize`(1)
 - [ ] `CutCornerShape`(5)
 - [x] `TextFieldBuffer.delete`(2) → TextFieldBuffer.* extension
-- [ ] `PointerInputScope.detectDragGestures`(7)
+- [x] `PointerInputScope.detectDragGestures`(7) → PointerInputScope.* extension
 - [ ] `PointerInputScope.detectDragGesturesAfterLongPress`(5)
 - [ ] `PointerInputScope.detectHorizontalDragGestures`(5)
 - [x] `PointerInputScope.detectTapGestures`(5) → PointerInputScope.* extension
-- [ ] `PointerInputScope.detectTransformGestures`(3)
+- [x] `PointerInputScope.detectTransformGestures`(3) → PointerInputScope.* extension
 - [ ] `PointerInputScope.detectVerticalDragGestures`(5)
 - [ ] `StyleScope.disabled`(1) *(experimental)*
 - [ ] `AwaitPointerEventScope.drag`(3)
@@ -443,7 +443,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `LazyListScope.itemsIndexed`(5)
 - [ ] `LazyGridScope.itemsIndexed`(6)
 - [ ] `LazyStaggeredGridScope.itemsIndexed`(6)
-- [ ] `KeyboardActions`(1)
+- [x] `KeyboardActions`(1) → covered by `OutlinedTextField.KeyboardActions`
 - [ ] `LazyGridPrefetchStrategy`(1) *(experimental)*
 - [ ] `LazyLayoutCacheWindow`(3) *(experimental)*
 - [ ] `LazyLayoutKeyIndexMap`(3)
@@ -1427,7 +1427,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 
 ## `ui`
 
-### @Composable functions — 10/26 (38%)
+### @Composable functions — 11/26 (42%)
 
 - [x] `AndroidView`(6) → type match
 - [x] `booleanResource`(1) → covered by `ComposeExtensions.BooleanResource`
@@ -1439,7 +1439,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [x] `integerArrayResource`(1) → covered by `ComposeExtensions.IntegerArrayResource`
 - [x] `integerResource`(1) → covered by `ComposeExtensions.IntegerResource`
 - [ ] `InterceptPlatformTextInput`(3) *(experimental)*
-- [ ] `Layout`(4)
+- [x] `Layout`(4) → type match
 - [ ] `LookaheadScope`(1)
 - [ ] `mediaQuery`(1) *(experimental)*
 - [x] `painterResource`(1) → covered by `ComposeExtensions.PainterResource`
@@ -1456,7 +1456,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `SubcomposeLayout`(4)
 - [ ] `TestModifierUpdaterLayout`(1)
 
-### Other top-level functions — 20/224 (9%)
+### Other top-level functions — 22/224 (10%)
 
 - [ ] `addPathNodes`(1)
 - [ ] `VelocityTracker.addPointerInputChange`(2)
@@ -1538,7 +1538,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [x] `Modifier.graphicsLayer`(20) → Modifier extension
 - [ ] `GraphicsLayerScope`(0)
 - [ ] `ImageVector.Builder.group`(11)
-- [ ] `SemanticsPropertyReceiver.heading`(0)
+- [x] `SemanticsPropertyReceiver.heading`(0) → SemanticsPropertyReceiver.* extension
 - [ ] `SemanticsPropertyReceiver.hideFromAccessibility`(0)
 - [ ] `ImageBitmap.Companion.imageResource`(2)
 - [ ] `SemanticsPropertyReceiver.indexForKey`(1)
@@ -1565,7 +1565,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `PointerButtons.isPressed`(1)
 - [ ] `Modifier.keepScreenOn`(0)
 - [ ] `Key`(1)
-- [ ] `Modifier.layout`(1)
+- [x] `Modifier.layout`(1) → Modifier extension
 - [ ] `Modifier.layoutBounds`(1)
 - [ ] `lerp`(3)
 - [ ] `LookaheadScope.lookaheadScopeCoordinates`(1)
@@ -1767,7 +1767,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `VerticalRuler`
 - [ ] `VNode`
 
-### Interfaces — 5/125 (4%)
+### Interfaces — 8/125 (6%)
 
 - [ ] `AccessibilityManager`
 - [x] `Alignment` → type match
@@ -1831,12 +1831,12 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `LayoutModifier`
 - [ ] `LayoutModifierNode`
 - [ ] `LookaheadScope`
-- [ ] `Measurable`
+- [x] `Measurable` → type match
 - [ ] `Measured`
 - [ ] `MeasuredSizeAwareModifierNode`
 - [ ] `MeasurePolicy`
-- [ ] `MeasureResult`
-- [ ] `MeasureScope`
+- [x] `MeasureResult` → type match
+- [x] `MeasureScope` → type match
 - [x] `Modifier` → type match
 - [ ] `ModifierLocalConsumer`
 - [ ] `ModifierLocalModifierNode`
@@ -1938,7 +1938,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `TouchBoundsExpansion`
 - [x] `TransformOrigin` → type match
 
-### Top-level properties — 6/106 (6%)
+### Top-level properties — 7/106 (7%)
 
 - [ ] `SemanticsPropertyReceiver.accessibilityClassName`
 - [ ] `SemanticsPropertyReceiver.collectionInfo by SemanticsProperties.CollectionInfo`
@@ -1977,7 +1977,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [ ] `FirstBaseline`
 - [ ] `SemanticsPropertyReceiver.focused by SemanticsProperties.Focused`
 - [ ] `SemanticsPropertyReceiver.horizontalScrollAxisRange`
-- [ ] `SemanticsPropertyReceiver.imeAction by SemanticsProperties.ImeAction`
+- [x] `SemanticsPropertyReceiver.imeAction by SemanticsProperties.ImeAction` → type match
 - [ ] `SemanticsPropertyReceiver.inputText by SemanticsProperties.InputText`
 - [ ] `SemanticsPropertyReceiver.inputTextSuggestionState`
 - [ ] `SemanticsPropertyReceiver.isContainer by SemanticsProperties.IsContainer`
@@ -2383,7 +2383,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 
 - [ ] `ResolvedTextDirection`
 
-### Value classes — 4/17 (24%)
+### Value classes — 5/17 (29%)
 
 - [ ] `BaselineShift`
 - [ ] `DeviceFontFamilyName`
@@ -2392,7 +2392,7 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 - [x] `FontStyle` → type match
 - [ ] `FontSynthesis`
 - [ ] `Hyphens`
-- [ ] `ImeAction`
+- [x] `ImeAction` → type match
 - [ ] `KeyboardCapitalization`
 - [x] `KeyboardType` → type match
 - [ ] `LineBreak`
@@ -2405,16 +2405,16 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 
 ## `ui-unit`
 
-### Other top-level functions — 1/22 (5%)
+### Other top-level functions — 2/22 (9%)
 
 - [ ] `Dp.coerceAtLeast`(1) → receiver Dp present, ext missing
 - [ ] `Dp.coerceAtMost`(1) → receiver Dp present, ext missing
 - [ ] `Dp.coerceIn`(2) → receiver Dp present, ext missing
-- [ ] `Constraints.constrain`(1)
-- [ ] `Constraints`(5)
+- [ ] `Constraints.constrain`(1) → receiver Constraints present, ext missing
+- [x] `Constraints`(5) → type match
 - [ ] `Density`(2)
 - [ ] `IntRect`(2)
-- [ ] `Constraints.isSatisfiedBy`(1)
+- [ ] `Constraints.isSatisfiedBy`(1) → receiver Constraints present, ext missing
 - [ ] `lerp`(3)
 - [x] `Constraints.offset`(2) → Constraints.* extension
 - [ ] `Rect.roundToIntRect`(0)
@@ -2452,9 +2452,9 @@ bumping a Compose package version. Coverage is symbol-level (short-name match).
 
 - [ ] `LayoutDirection`
 
-### Value classes — 1/9 (11%)
+### Value classes — 2/9 (22%)
 
-- [ ] `Constraints`
+- [x] `Constraints` → type match
 - [x] `Dp` → type match
 - [ ] `DpOffset`
 - [ ] `DpSize`
@@ -2550,6 +2550,7 @@ a Compose API the matcher didn't pair up.
 - `AndroidX.Compose.CompositionLocal<T>`
 - `AndroidX.Compose.CustomTab`
 - `AndroidX.Compose.DateRangePickerDialog`
+- `AndroidX.Compose.DateRangeSelectableDates`
 - `AndroidX.Compose.Icons.AutoMirrored.Default`
 - `AndroidX.Compose.Icons.Default`
 - `AndroidX.Compose.DerivedState<T>`
@@ -2562,6 +2563,7 @@ a Compose API the matcher didn't pair up.
 - `AndroidX.Compose.Icons`
 - `AndroidX.Compose.IState<T>`
 - `AndroidX.Compose.IStateFlow<T>`
+- `AndroidX.Compose.KeyboardActionsHelper`
 - `AndroidX.Compose.KeyboardOptionsCompanion`
 - `AndroidX.Compose.LargeFlexibleTopAppBar`
 - `AndroidX.Compose.LocalColorScheme`
@@ -2577,6 +2579,7 @@ a Compose API the matcher didn't pair up.
 - `AndroidX.Compose.NavOptions`
 - `AndroidX.Compose.Icons.AutoMirrored.Outlined`
 - `AndroidX.Compose.Icons.Outlined`
+- `AndroidX.Compose.PlacementScope`
 - `AndroidX.Compose.Resource`
 - `AndroidX.Compose.Icons.AutoMirrored.Rounded`
 - `AndroidX.Compose.Icons.Rounded`

samples/JetNews/InterestsScreen.cs

@@ -1,10 +1,12 @@
+using Placeable = AndroidX.Compose.UI.Layout.Placeable;
+
 namespace AndroidX.Compose.Samples.JetNews;
 
 /// <summary>
 /// JetNews interests screen — a <see cref="PrimaryTabRow"/> with three
 /// tabs (Topics / People / Publications) and a per-tab toggleable
-/// list. The upstream sample renders Topics in an adaptive two-column
-/// custom <c>Layout</c> on wide screens; we render a single column.
+/// list. Topics are rendered through a custom <see cref="Layout"/>
+/// that re-balances rows across two columns on wide screens.
 /// </summary>
 public static class InterestsScreen
 {
@@ -48,7 +50,7 @@ static Column BuildBody(
             },
             selectedTab.Value switch
             {
-                0 => (ComposableNode) BuildTopics(selectedTopics),
+                0 => BuildTopics(selectedTopics),
                 1 => BuildSimpleList(InterestsRepo.People, selectedPeople),
                 _ => BuildSimpleList(InterestsRepo.Publications, selectedPublications),
             },
@@ -62,23 +64,88 @@ static Tab BuildTab(MutableState<int> selectedTab, int index, string label) =>
             Text = new Text(label) { FontSize = 14 },
         };
 
-    static Column BuildTopics(MutableStateList<string> selected)
+    static ComposableNode BuildTopics(MutableStateList<string> selected) =>
+        new Composed(c =>
+        {
+            // Topics content can exceed viewport height (especially landscape),
+            // so the outer column needs vertical scrolling.
+            var scroll = c.Remember(() => new ScrollState());
+            var col = new Column
+            {
+                Modifier.FillMaxWidth().VerticalScroll(scroll),
+            };
+            foreach (var section in InterestsRepo.Topics)
+            {
+                col.Add(BuildSectionHeader(section.Key));
+                var rows = new List<ComposableNode>();
+                foreach (var topic in section.Value)
+                {
+                    var key = $"{section.Key}/{topic}";
+                    rows.Add(BuildToggleRow(topic, selected.Contains(key), () => Toggle(selected, key)));
+                }
+                col.Add(BuildAdaptiveTopicSection(rows));
+                col.Add(new HorizontalDivider
+                {
+                    Modifier = Modifier.Padding(horizontal: 16, vertical: 8),
+                });
+            }
+            return col;
+        });
+
+    // Mirrors upstream JetNews's `InterestsAdaptiveContentLayout` — bins
+    // topic rows into N columns whose count is driven by the parent's
+    // available width. Backed by Compose's low-level `Layout` primitive.
+    static Layout BuildAdaptiveTopicSection(IReadOnlyList<ComposableNode> rows)
     {
-        var col = new Column { Modifier.FillMaxWidth() };
-        foreach (var section in InterestsRepo.Topics)
+        const int multiColumnBreakpointPx = 600;
+
+        var layout = new Layout(measurePolicy: (scope, measurables, constraints) =>
         {
-            col.Add(BuildSectionHeader(section.Key));
-            foreach (var topic in section.Value)
+            int max = constraints.HasBoundedWidth
+                ? constraints.MaxWidth
+                : multiColumnBreakpointPx;
+            int columns = max >= multiColumnBreakpointPx ? 2 : 1;
+            int columnWidth = max / columns;
+
+            var childConstraints = Constraints.FixedWidth(columnWidth);
+            var placeables       = new Placeable[measurables.Count];
+            var columnHeights    = new int[columns];
+            var columnAssign     = new int[measurables.Count];
+            for (int i = 0; i < measurables.Count; i++)
             {
-                var key = $"{section.Key}/{topic}";
-                col.Add(BuildToggleRow(topic, selected.Contains(key), () => Toggle(selected, key)));
+                placeables[i] = measurables[i].Measure(childConstraints);
+                int h = placeables[i].Height;
+                int target = 0;
+                for (int c = 1; c < columns; c++)
+                    if (columnHeights[c] < columnHeights[target]) target = c;
+                columnAssign[i] = target;
+                columnHeights[target] += h;
             }
-            col.Add(new HorizontalDivider
+
+            int totalHeight = 0;
+            for (int c = 0; c < columns; c++)
+                if (columnHeights[c] > totalHeight) totalHeight = columnHeights[c];
+
+            return scope.Layout(columnWidth * columns, totalHeight, placement =>
             {
-                Modifier = Modifier.Padding(horizontal: 16, vertical: 8),
+                var yByColumn = new int[columns];
+                for (int i = 0; i < placeables.Length; i++)
+                {
+                    int col = columnAssign[i];
+                    placement.PlaceRelative(
+                        placeables[i],
+                        x: col * columnWidth,
+                        y: yByColumn[col]);
+                    yByColumn[col] += placeables[i].Height;
+                }
             });
-        }
-        return col;
+        })
+        {
+            Modifier.FillMaxWidth(),
+        };
+        foreach (var row in rows)
+            layout.Add(row);
+        return layout;
     }
 
     static LazyColumn<string> BuildSimpleList(IReadOnlyList<string> items,