Empty change-with-nth restores the default --with-nth value

Previously, change-with-nth() with an empty value cleared with-nth
entirely. Now it restores the original --with-nth expression, matching
the convention used by change-nth.

src/terminal.go

@@ -340,6 +340,7 @@ type Terminal struct {
 	nthAttr              tui.Attr
 	nth                  []Range
 	nthCurrent           []Range
+	withNthDefault       string
 	withNthExpr          string
 	withNthEnabled       bool
 	acceptNth            func([]Token, int32) string
@@ -1093,6 +1094,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox, executor *util.Executor
 		nthAttr:            opts.Theme.Nth.Attr,
 		nth:                opts.Nth,
 		nthCurrent:         opts.Nth,
+		withNthDefault:     opts.WithNthExpr,
 		withNthExpr:        opts.WithNthExpr,
 		withNthEnabled:     opts.WithNth != nil,
 		tabstop:            opts.Tabstop,
@@ -6386,10 +6388,12 @@ func (t *Terminal) Loop() error {
 					if len(tokens) > 1 {
 						a.a = strings.Join(append(tokens[1:], tokens[0]), "|")
 					}
+					// Empty value restores the default --with-nth
+					if len(withNthExpr) == 0 {
+						withNthExpr = t.withNthDefault
+					}
 					if withNthExpr != t.withNthExpr {
-						if len(withNthExpr) == 0 {
-							newWithNth = &withNthSpec{fn: nil}
-						} else if factory, err := nthTransformer(withNthExpr); err == nil {
+						if factory, err := nthTransformer(withNthExpr); err == nil {
 							newWithNth = &withNthSpec{fn: factory(t.delimiter)}
 						} else {
 							return

test/test_core.rb

@@ -1780,16 +1780,25 @@ class TestCore < TestInteractive
     end
   end
 
-  def test_change_with_nth_clear
-    tmux.send_keys %(echo -e 'a b c\nd e f' | #{FZF} --with-nth 1 --bind 'space:change-with-nth()'), :Enter
+  def test_change_with_nth_default
+    # Empty value restores the default --with-nth
+    tmux.send_keys %(echo -e 'a b c\nd e f' | #{FZF} --with-nth 1 --bind 'space:change-with-nth(2|)'), :Enter
     tmux.until do |lines|
       assert_equal 2, lines.item_count
       assert lines.any_include?('a')
       refute lines.any_include?('b')
     end
+    # Switch to field 2
     tmux.send_keys :Space
     tmux.until do |lines|
-      assert lines.any_include?('a b c')
+      assert lines.any_include?('b')
+      refute lines.any_include?('a')
+    end
+    # Empty restores default (field 1)
+    tmux.send_keys :Space
+    tmux.until do |lines|
+      assert lines.any_include?('a')
+      refute lines.any_include?('b')
     end
   end