Properly measure Box children (#2398)

The Kotlin syntax I was using was not working. I can't
say why.

Closes: #2394

CHANGELOG.md

@@ -14,6 +14,7 @@ Fixed:
 - Fix a layout bug where children of fixed-with `Row` containers were assigned the wrong width.
 - Fix inconsistencies between iOS and Android for `Column` and `Row` layouts.
 - Correctly update the layout when a Box's child's modifiers are removed.
+- Fix a layout bug where children of `Box` containers were not measured properly.
 
 Breaking:
 - Replace `CodeListener` with new functions in `RedwoodView`. This puts the loading/error/ready state with the UI that displays that state.

redwood-layout-shared-test/src/commonMain/kotlin/app/cash/redwood/layout/AbstractBoxTest.kt

@@ -554,4 +554,36 @@ abstract class AbstractBoxTest<T : Any> {
     b.text("BBB_v2")
     snapshotter.snapshot("v2")
   }
+
+  @Test
+  fun testChildExplicitHeight() {
+    val container = box()
+      .apply {
+        width(Constraint.Fill)
+        height(Constraint.Fill)
+        horizontalAlignment(CrossAxisAlignment.Stretch)
+        verticalAlignment(CrossAxisAlignment.Center)
+      }
+    val snapshotter = snapshotter(container.value)
+
+    val box = box()
+      .apply {
+        modifier = MarginImpl(all = 24.dp)
+        width(Constraint.Fill)
+        height(Constraint.Wrap)
+        horizontalAlignment(CrossAxisAlignment.Stretch)
+        verticalAlignment(CrossAxisAlignment.Center)
+      }
+      .also { container.children.insert(0, it) }
+
+    val a = widgetFactory.color().apply {
+      modifier = HeightImpl(100.dp)
+    }.also { box.children.insert(0, it) }
+
+    val b = widgetFactory.text(
+      text = "foreground",
+    ).also { box.children.insert(1, it) }
+
+    snapshotter.snapshot()
+  }
 }

redwood-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/UIViewBox.kt

@@ -341,14 +341,22 @@ internal fun CGSizeMake(
 
 /** Returns the smallest size that wraps both [a] and [b]. */
 internal fun maxEachDimension(a: CValue<CGSize>, b: CValue<CGSize>): CValue<CGSize> {
-  return [email protected] {
-    [email protected] {
-      CGSizeMake(
-        maxOf(a@width, b@width),
-        maxOf(a@height, b@height),
-      )
-    }
+  val aWidth: CGFloat
+  val aHeight: CGFloat
+  a.useContents {
+    aWidth = width
+    aHeight = height
+  }
+  val bWidth: CGFloat
+  val bHeight: CGFloat
+  b.useContents {
+    bWidth = width
+    bHeight = height
   }
+  return CGSizeMake(
+    maxOf(aWidth, bWidth),
+    maxOf(aHeight, bHeight),
+  )
 }
 
 /** Returns a margin that uses the largest of [a] and [b] for each side. */