[Carousel] Update multi-browse strategy to always have at least 1 medium item

PiperOrigin-RevId: 719372980

Author: imhappi

lib/java/com/google/android/material/carousel/MultiBrowseCarouselStrategy.java

@@ -95,7 +95,7 @@ public KeylineState onFirstChildMeasuredWithMargins(
     // then finally 1.
 
     int[] smallCounts = SMALL_COUNTS;
-    if (carouselSize < smallChildSizeMin * 2) {
+    if (carouselSize <= smallChildSizeMin * 2) {
       // If the available space is too small to fit a large item and small item and a large item
       // (large items must be at least as big as a small item), allow arrangements with no small
       // items.
@@ -134,7 +134,19 @@ public KeylineState onFirstChildMeasuredWithMargins(
 
     keylineCount = arrangement.getItemCount();
 
-    if (ensureArrangementFitsItemCount(arrangement, carousel.getItemCount())) {
+    boolean refreshArrangement =
+        ensureArrangementFitsItemCount(arrangement, carousel.getItemCount());
+
+    // Ensure that the arrangement is never only filled with large items unless there's no space for
+    // 2 small item.
+    if (arrangement.mediumCount == 0
+        && arrangement.smallCount == 0
+        && carouselSize > 2 * smallChildSizeMin) {
+      arrangement.smallCount = 1;
+      refreshArrangement = true;
+    }
+
+    if (refreshArrangement) {
       // In case counts changed after ensuring the previous arrangement fit the item
       // counts, we call `findLowestCostArrangement` again with the item counts set.
       arrangement =

lib/javatests/com/google/android/material/carousel/MultiBrowseCarouselStrategyTest.java

@@ -95,6 +95,22 @@ public void testSmallContainer_shouldShowOneLargeItem() {
     assertThat(keylineState.getKeylines().get(1).maskedItemSize).isEqualTo((float) carouselWidth);
   }
 
+  @Test
+  public void testContainer_shouldShowOneLargeOneSmallItem() {
+    View view = createViewWithSize(ApplicationProvider.getApplicationContext(), 100, 400);
+    float minSmallItemSize =
+        view.getResources().getDimension(R.dimen.m3_carousel_small_item_size_min);
+    // Create a carousel that will fit at least one small item and one larger item
+    int carouselWidth = ((int) (minSmallItemSize * 2f)) + 1;
+    Carousel carousel = createCarouselWithWidth(carouselWidth);
+
+    MultiBrowseCarouselStrategy config = setupStrategy();
+    KeylineState keylineState = config.onFirstChildMeasuredWithMargins(carousel, view);
+
+    assertThat(keylineState.getKeylines()).hasSize(4);
+    assertThat(keylineState.getKeylines().get(2).maskedItemSize).isEqualTo(minSmallItemSize);
+  }
+
   @Test
   public void testKnownArrangementWithMediumItem_correctlyCalculatesKeylineLocations() {
     float[] locOffsets = new float[] {-.5F, 100F, 300F, 464F, 556F, 584.5F};