Fix nondeterministic highlight positions in FuzzyMatchV2 backtrace

Phase 3 writes C rows only from column F[r], but tie-breaking in the
backtrace could read row i+1 at column j+1 left of F[i+1]. With reused
slab, that cell holds data from previously processed item, so highlight
positions in equal-score ties depended on processing order.

src/algo/algo.go

@@ -630,6 +630,7 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 				s2 = H[I+j0-1]
 			}
 
+			row := i
 			if s > s1 && (s > s2 || s == s2 && preferMatch) {
 				*pos = append(*pos, j+minIdx)
 				if i == 0 {
@@ -637,7 +638,10 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 				}
 				i--
 			}
-			preferMatch = C[I+j0] > 1 || I+width+j0+1 < len(C) && C[I+width+j0+1] > 0
+			// Row below is only written from column F[row+1]; don't read
+			// stale slab data left of it
+			preferMatch = C[I+j0] > 1 ||
+				row+1 < M && j < lastIdx && int32(j+1) >= F[row+1] && C[I+width+j0+1] > 0
 			j--
 		}
 	}

src/algo/algo_test.go

@@ -218,3 +218,26 @@ func TestLongStringWithNormalize(t *testing.T) {
 	unicodeString := string(bytes) + " Minímal example"
 	assertMatch2(t, FuzzyMatchV1, false, true, false, unicodeString, "minim", 30001, 30006, 140)
 }
+
+func TestResultPositionsWithReusedSlab(t *testing.T) {
+	// Backtrace positions in equal-score ties must not depend on data
+	// a previous match left in the slab
+	pattern := []rune("co/")
+	target := util.ToChars([]byte("core_color/view/server.txt"))
+	_, freshPos := FuzzyMatchV2(false, false, true, &target, pattern, true, util.MakeSlab(100*1024, 2048))
+
+	slab := util.MakeSlab(100*1024, 2048)
+	dirty := util.ToChars([]byte("completion/keybinding/client/handler/writer_index.txt"))
+	FuzzyMatchV2(false, false, true, &dirty, pattern, true, slab)
+	_, reusedPos := FuzzyMatchV2(false, false, true, &target, pattern, true, slab)
+
+	if len(*freshPos) != len(*reusedPos) {
+		t.Fatalf("position count mismatch: %v vs %v", *freshPos, *reusedPos)
+	}
+	for i := range *freshPos {
+		if (*freshPos)[i] != (*reusedPos)[i] {
+			t.Errorf("positions differ with reused slab: %v vs %v", *freshPos, *reusedPos)
+			break
+		}
+	}
+}