Fix --no-sort jumpiness by merging results in index order

Fix #4717

src/merger.go

@@ -136,14 +136,7 @@ func (mg *Merger) Get(idx int) Result {
 	if mg.tac {
 		idx = mg.count - idx - 1
 	}
-	for _, list := range mg.lists {
-		numItems := len(list)
-		if idx < numItems {
-			return list[idx]
-		}
-		idx -= numItems
-	}
-	panic(fmt.Sprintf("Index out of bounds (unsorted, %d/%d)", idx, mg.count))
+	return mg.mergedGet(idx)
 }
 
 func (mg *Merger) ToMap() map[int32]Result {
@@ -171,7 +164,7 @@ func (mg *Merger) mergedGet(idx int) Result {
 			}
 			if cursor >= 0 {
 				rank := list[cursor]
-				if minIdx < 0 || compareRanks(rank, minRank, mg.tac) {
+				if minIdx < 0 || mg.sorted && compareRanks(rank, minRank, mg.tac) || !mg.sorted && rank.item.Index() < minRank.item.Index() {
 					minRank = rank
 					minIdx = listIdx
 				}

src/merger_test.go

@@ -54,10 +54,25 @@ func buildLists(partiallySorted bool) ([][]Result, []Result) {
 }
 
 func TestMergerUnsorted(t *testing.T) {
-	lists, items := buildLists(false)
+	lists, _ := buildLists(false)
+
+	// Sort each list by index to simulate real worker behavior
+	// (workers process chunks in ascending order via nextChunk.Add(1))
+	for _, list := range lists {
+		sort.Slice(list, func(i, j int) bool {
+			return list[i].item.Index() < list[j].item.Index()
+		})
+	}
+	items := []Result{}
+	for _, list := range lists {
+		items = append(items, list...)
+	}
+	sort.Slice(items, func(i, j int) bool {
+		return items[i].item.Index() < items[j].item.Index()
+	})
 	cnt := len(items)
 
-	// Not sorted: same order
+	// Not sorted: items in ascending index order
 	mg := NewMerger(nil, lists, false, false, revision{}, 0, 0)
 	assert(t, cnt == mg.Length(), "Invalid Length")
 	for i := range cnt {