tpl/collections: Fix append when appending a slice to a slice of slices

Fixes gohugoio#11004

common/collections/append.go

@@ -60,13 +60,28 @@ func Append(to any, from ...any) (any, error) {
 		return Slice(from...), nil
 	}
 
+	// See issue #11004.
+	isToASliceOfSlices := tov.Len() > 0
+	for i := 0; i < tov.Len(); i++ {
+		v, isNil := indirect(tov.Index(i))
+		if isNil || v.Kind() != reflect.Slice {
+			isToASliceOfSlices = false
+			break
+		}
+	}
+
+	if isToASliceOfSlices {
+		return reflect.Append(tov, reflect.ValueOf(from)).Interface(), nil
+	}
+
 	for _, f := range from {
 		fv := reflect.ValueOf(f)
 		if !fv.Type().AssignableTo(tot) {
 			// Fall back to a []interface{} slice.
 			tov, _ := indirect(reflect.ValueOf(to))
 			return appendToInterfaceSlice(tov, from...)
 		}
+
 		tov = reflect.Append(tov, fv)
 	}
 

tpl/collections/append_test.go

@@ -34,6 +34,10 @@ func TestAppend(t *testing.T) {
 		{[]string{"a", "b"}, []any{"c"}, []string{"a", "b", "c"}},
 		{[]string{"a", "b"}, []any{"c", "d", "e"}, []string{"a", "b", "c", "d", "e"}},
 		{[]string{"a", "b"}, []any{[]string{"c", "d", "e"}}, []string{"a", "b", "c", "d", "e"}},
+		// Issue #11004
+		{[]any{[]any{"a"}}, []any{"b"}, []any{[]any{"a"}, []any{"b"}}},
+		{[]any{[]any{"a"}, "b"}, []any{"c"}, []any{[]any{"a"}, "b", "c"}},
+
 		// Errors
 		{"", []any{[]string{"a", "b"}}, false},
 		{[]string{"a", "b"}, []any{}, false},