home: reveal new front items and brick-pack genre chips

Scroll a recently played row back to the front when a new first item
appears while it is already at/near the start; the lazy layout otherwise
anchors the old first item and hides the new one. Balance the three
genre rows by estimated width and tighten their gap.

app/src/test/kotlin/com/subtracks/ui/home/GenreRowsTest.kt

@@ -0,0 +1,41 @@
+package com.subtracks.ui.home
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class GenreRowsTest {
+    private fun weight(genre: String) = genre.length + 6
+
+    @Test
+    fun everyGenreIsPlacedExactlyOnce() {
+        val genres = listOf("Electronic", "Rock", "Pop", "Jazz", "Classical", "Ambient")
+
+        val rows = brickRows(genres, 3)
+
+        assertEquals(3, rows.size)
+        assertEquals(genres.sorted(), rows.flatten().sorted())
+    }
+
+    @Test
+    fun rowsBalanceByWidthNotCount() {
+        val genres =
+            listOf(
+                "VeryLongGenreName1",
+                "A",
+                "B",
+                "VeryLongGenreName2",
+                "C",
+                "D",
+                "VeryLongGenreName3",
+                "E",
+                "F",
+            )
+
+        val rows = brickRows(genres, 3)
+
+        val widths = rows.map { row -> row.sumOf(::weight) }
+        val largestChip = genres.maxOf(::weight)
+        assertTrue(widths.max() - widths.min() <= largestChip)
+    }
+}

app/src/test/kotlin/com/subtracks/ui/home/HomeRevealScrollTest.kt

@@ -0,0 +1,56 @@
+package com.subtracks.ui.home
+
+import androidx.activity.ComponentActivity
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.lazy.LazyListState
+import androidx.compose.foundation.lazy.LazyRow
+import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.lazy.rememberLazyListState
+import androidx.compose.material3.Text
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.test.junit4.v2.createAndroidComposeRule
+import androidx.compose.ui.unit.dp
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.github.takahirom.roborazzi.RobolectricDeviceQualifiers
+import com.subtracks.ui.theme.SubtracksTheme
+import org.junit.Assert.assertEquals
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.annotation.Config
+import org.robolectric.annotation.GraphicsMode
+
+@RunWith(AndroidJUnit4::class)
+@GraphicsMode(GraphicsMode.Mode.NATIVE)
+@Config(qualifiers = RobolectricDeviceQualifiers.PixelXL)
+class HomeRevealScrollTest {
+    @get:Rule
+    val composeRule = createAndroidComposeRule<ComponentActivity>()
+
+    @Test
+    fun aNewFrontItemAtTheStartScrollsBack() {
+        val labels = mutableStateOf(listOf("a", "b", "c", "d", "e"))
+        lateinit var state: LazyListState
+        composeRule.setContent {
+            SubtracksTheme {
+                state = rememberLazyListState()
+                RevealNewFrontItem(labels.value, state) { it }
+                LazyRow(state = state, modifier = Modifier.fillMaxSize()) {
+                    items(labels.value, key = { it }) { label ->
+                        Text(label, modifier = Modifier.width(200.dp).height(80.dp))
+                    }
+                }
+            }
+        }
+        composeRule.waitForIdle()
+        assertEquals(0, state.firstVisibleItemIndex)
+
+        composeRule.runOnIdle { labels.value = listOf("z") + labels.value }
+
+        composeRule.waitForIdle()
+        assertEquals(0, state.firstVisibleItemIndex)
+    }
+}

app/src/test/kotlin/com/subtracks/ui/home/HomeRevealTest.kt

@@ -0,0 +1,37 @@
+package com.subtracks.ui.home
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class HomeRevealTest {
+    @Test
+    fun noPreviousFrontDoesNotReveal() {
+        assertFalse(shouldRevealNewFront(previousKey = null, firstKey = "a", firstVisibleItemIndex = 0))
+    }
+
+    @Test
+    fun noFrontItemDoesNotReveal() {
+        assertFalse(shouldRevealNewFront(previousKey = "a", firstKey = null, firstVisibleItemIndex = 0))
+    }
+
+    @Test
+    fun anUnchangedFrontDoesNotReveal() {
+        assertFalse(shouldRevealNewFront(previousKey = "a", firstKey = "a", firstVisibleItemIndex = 0))
+    }
+
+    @Test
+    fun aChangedFrontRevealsAtTheStart() {
+        assertTrue(shouldRevealNewFront(previousKey = "a", firstKey = "b", firstVisibleItemIndex = 0))
+    }
+
+    @Test
+    fun aChangedFrontRevealsOneRowIn() {
+        assertTrue(shouldRevealNewFront(previousKey = "a", firstKey = "b", firstVisibleItemIndex = 1))
+    }
+
+    @Test
+    fun aChangedFrontDoesNotRevealWhenScrolledAway() {
+        assertFalse(shouldRevealNewFront(previousKey = "a", firstKey = "b", firstVisibleItemIndex = 2))
+    }
+}