fix(cli): make stdio routing injectable

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/cli/app.go

@@ -7,9 +7,9 @@ import (
 	"os"
 	"runtime/debug"
 
+	"dappco.re/go/core"
 	"forge.lthn.ai/core/go-i18n"
 	"forge.lthn.ai/core/go-log"
-	"dappco.re/go/core"
 	"github.com/spf13/cobra"
 )
 
@@ -175,13 +175,13 @@ PowerShell:
 		Run: func(cmd *cobra.Command, args []string) {
 			switch args[0] {
 			case "bash":
-				_ = cmd.Root().GenBashCompletion(os.Stdout)
+				_ = cmd.Root().GenBashCompletion(stdoutWriter())
 			case "zsh":
-				_ = cmd.Root().GenZshCompletion(os.Stdout)
+				_ = cmd.Root().GenZshCompletion(stdoutWriter())
 			case "fish":
-				_ = cmd.Root().GenFishCompletion(os.Stdout, true)
+				_ = cmd.Root().GenFishCompletion(stdoutWriter(), true)
 			case "powershell":
-				_ = cmd.Root().GenPowerShellCompletionWithDesc(os.Stdout)
+				_ = cmd.Root().GenPowerShellCompletionWithDesc(stdoutWriter())
 			}
 		},
 	}

pkg/cli/daemon.go

@@ -60,7 +60,10 @@ func DetectMode() Mode {
 //	    cli.Success("interactive output enabled")
 //	}
 func IsTTY() bool {
-	return term.IsTerminal(int(os.Stdout.Fd()))
+	if f, ok := stdoutWriter().(*os.File); ok {
+		return term.IsTerminal(int(f.Fd()))
+	}
+	return false
 }
 
 // IsStdinTTY returns true if stdin is a terminal.
@@ -69,7 +72,10 @@ func IsTTY() bool {
 //	    cli.Warn("input is piped")
 //	}
 func IsStdinTTY() bool {
-	return term.IsTerminal(int(os.Stdin.Fd()))
+	if f, ok := stdinReader().(*os.File); ok {
+		return term.IsTerminal(int(f.Fd()))
+	}
+	return false
 }
 
 // IsStderrTTY returns true if stderr is a terminal.
@@ -78,5 +84,8 @@ func IsStdinTTY() bool {
 //	    cli.Progress("load", 1, 3, "config")
 //	}
 func IsStderrTTY() bool {
-	return term.IsTerminal(int(os.Stderr.Fd()))
+	if f, ok := stderrWriter().(*os.File); ok {
+		return term.IsTerminal(int(f.Fd()))
+	}
+	return false
 }

pkg/cli/errors.go

@@ -120,7 +120,7 @@ func Exit(code int, err error) error {
 func Fatal(err error) {
 	if err != nil {
 		LogError("Fatal error", "err", err)
-		fmt.Fprintln(os.Stderr, ErrorStyle.Render(Glyph(":cross:")+" "+err.Error()))
+		fmt.Fprintln(stderrWriter(), ErrorStyle.Render(Glyph(":cross:")+" "+err.Error()))
 		os.Exit(1)
 	}
 }
@@ -131,7 +131,7 @@ func Fatal(err error) {
 func Fatalf(format string, args ...any) {
 	msg := fmt.Sprintf(format, args...)
 	LogError("Fatal error", "msg", msg)
-	fmt.Fprintln(os.Stderr, ErrorStyle.Render(Glyph(":cross:")+" "+msg))
+	fmt.Fprintln(stderrWriter(), ErrorStyle.Render(Glyph(":cross:")+" "+msg))
 	os.Exit(1)
 }
 
@@ -147,7 +147,7 @@ func FatalWrap(err error, msg string) {
 	}
 	LogError("Fatal error", "msg", msg, "err", err)
 	fullMsg := fmt.Sprintf("%s: %v", msg, err)
-	fmt.Fprintln(os.Stderr, ErrorStyle.Render(Glyph(":cross:")+" "+fullMsg))
+	fmt.Fprintln(stderrWriter(), ErrorStyle.Render(Glyph(":cross:")+" "+fullMsg))
 	os.Exit(1)
 }
 
@@ -164,6 +164,6 @@ func FatalWrapVerb(err error, verb, subject string) {
 	msg := i18n.ActionFailed(verb, subject)
 	LogError("Fatal error", "msg", msg, "err", err, "verb", verb, "subject", subject)
 	fullMsg := fmt.Sprintf("%s: %v", msg, err)
-	fmt.Fprintln(os.Stderr, ErrorStyle.Render(Glyph(":cross:")+" "+fullMsg))
+	fmt.Fprintln(stderrWriter(), ErrorStyle.Render(Glyph(":cross:")+" "+fullMsg))
 	os.Exit(1)
 }

pkg/cli/frame.go

@@ -61,7 +61,7 @@ func NewFrame(variant string) *Frame {
 		variant: variant,
 		layout:  Layout(variant),
 		models:  make(map[Region]Model),
-		out:     os.Stderr,
+		out:     stderrWriter(),
 		done:    make(chan struct{}),
 		focused: RegionContent,
 		keyMap:  DefaultKeyMap(),
@@ -467,7 +467,7 @@ func (f *Frame) runLive() {
 	opts := []tea.ProgramOption{
 		tea.WithAltScreen(),
 	}
-	if f.out != os.Stdout {
+	if f.out != stdoutWriter() {
 		opts = append(opts, tea.WithOutput(f.out))
 	}
 

pkg/cli/io.go

@@ -0,0 +1,68 @@
+package cli
+
+import (
+	"io"
+	"os"
+	"sync"
+)
+
+var (
+	stdin io.Reader = os.Stdin
+
+	stdoutOverride io.Writer
+	stderrOverride io.Writer
+
+	ioMu sync.RWMutex
+)
+
+// SetStdin overrides the default stdin reader for testing.
+// Pass nil to restore the real os.Stdin reader.
+func SetStdin(r io.Reader) {
+	ioMu.Lock()
+	defer ioMu.Unlock()
+	if r == nil {
+		stdin = os.Stdin
+		return
+	}
+	stdin = r
+}
+
+// SetStdout overrides the default stdout writer.
+// Pass nil to restore writes to os.Stdout.
+func SetStdout(w io.Writer) {
+	ioMu.Lock()
+	defer ioMu.Unlock()
+	stdoutOverride = w
+}
+
+// SetStderr overrides the default stderr writer.
+// Pass nil to restore writes to os.Stderr.
+func SetStderr(w io.Writer) {
+	ioMu.Lock()
+	defer ioMu.Unlock()
+	stderrOverride = w
+}
+
+func stdinReader() io.Reader {
+	ioMu.RLock()
+	defer ioMu.RUnlock()
+	return stdin
+}
+
+func stdoutWriter() io.Writer {
+	ioMu.RLock()
+	defer ioMu.RUnlock()
+	if stdoutOverride != nil {
+		return stdoutOverride
+	}
+	return os.Stdout
+}
+
+func stderrWriter() io.Writer {
+	ioMu.RLock()
+	defer ioMu.RUnlock()
+	if stderrOverride != nil {
+		return stderrOverride
+	}
+	return os.Stderr
+}

pkg/cli/output.go

@@ -2,7 +2,6 @@ package cli
 
 import (
 	"fmt"
-	"os"
 	"strings"
 
 	"forge.lthn.ai/core/go-i18n"
@@ -10,35 +9,35 @@ import (
 
 // Blank prints an empty line.
 func Blank() {
-	fmt.Println()
+	fmt.Fprintln(stdoutWriter())
 }
 
 // Echo translates a key via i18n.T and prints with newline.
 // No automatic styling - use Success/Error/Warn/Info for styled output.
 func Echo(key string, args ...any) {
-	fmt.Println(compileGlyphs(i18n.T(key, args...)))
+	fmt.Fprintln(stdoutWriter(), compileGlyphs(i18n.T(key, args...)))
 }
 
 // Print outputs formatted text (no newline).
 // Glyph shortcodes like :check: are converted.
 func Print(format string, args ...any) {
-	fmt.Print(compileGlyphs(fmt.Sprintf(format, args...)))
+	fmt.Fprint(stdoutWriter(), compileGlyphs(fmt.Sprintf(format, args...)))
 }
 
 // Println outputs formatted text with newline.
 // Glyph shortcodes like :check: are converted.
 func Println(format string, args ...any) {
-	fmt.Println(compileGlyphs(fmt.Sprintf(format, args...)))
+	fmt.Fprintln(stdoutWriter(), compileGlyphs(fmt.Sprintf(format, args...)))
 }
 
 // Text prints arguments like fmt.Println, but handling glyphs.
 func Text(args ...any) {
-	fmt.Println(compileGlyphs(fmt.Sprint(args...)))
+	fmt.Fprintln(stdoutWriter(), compileGlyphs(fmt.Sprint(args...)))
 }
 
 // Success prints a success message with checkmark (green).
 func Success(msg string) {
-	fmt.Println(SuccessStyle.Render(Glyph(":check:") + " " + compileGlyphs(msg)))
+	fmt.Fprintln(stdoutWriter(), SuccessStyle.Render(Glyph(":check:")+" "+compileGlyphs(msg)))
 }
 
 // Successf prints a formatted success message.
@@ -49,7 +48,7 @@ func Successf(format string, args ...any) {
 // Error prints an error message with cross (red) to stderr and logs it.
 func Error(msg string) {
 	LogError(msg)
-	fmt.Fprintln(os.Stderr, ErrorStyle.Render(Glyph(":cross:")+" "+compileGlyphs(msg)))
+	fmt.Fprintln(stderrWriter(), ErrorStyle.Render(Glyph(":cross:")+" "+compileGlyphs(msg)))
 }
 
 // Errorf prints a formatted error message to stderr and logs it.
@@ -86,7 +85,7 @@ func ErrorWrapAction(err error, verb string) {
 // Warn prints a warning message with warning symbol (amber) to stderr and logs it.
 func Warn(msg string) {
 	LogWarn(msg)
-	fmt.Fprintln(os.Stderr, WarningStyle.Render(Glyph(":warn:")+" "+compileGlyphs(msg)))
+	fmt.Fprintln(stderrWriter(), WarningStyle.Render(Glyph(":warn:")+" "+compileGlyphs(msg)))
 }
 
 // Warnf prints a formatted warning message to stderr and logs it.
@@ -96,7 +95,7 @@ func Warnf(format string, args ...any) {
 
 // Info prints an info message with info symbol (blue).
 func Info(msg string) {
-	fmt.Println(InfoStyle.Render(Glyph(":info:") + " " + compileGlyphs(msg)))
+	fmt.Fprintln(stdoutWriter(), InfoStyle.Render(Glyph(":info:")+" "+compileGlyphs(msg)))
 }
 
 // Infof prints a formatted info message.
@@ -106,7 +105,7 @@ func Infof(format string, args ...any) {
 
 // Dim prints dimmed text.
 func Dim(msg string) {
-	fmt.Println(DimStyle.Render(compileGlyphs(msg)))
+	fmt.Fprintln(stdoutWriter(), DimStyle.Render(compileGlyphs(msg)))
 }
 
 // Progress prints a progress indicator that overwrites the current line.
@@ -114,20 +113,20 @@ func Dim(msg string) {
 func Progress(verb string, current, total int, item ...string) {
 	msg := compileGlyphs(i18n.Progress(verb))
 	if len(item) > 0 && item[0] != "" {
-		fmt.Fprintf(os.Stderr, "\033[2K\r%s %d/%d %s", DimStyle.Render(msg), current, total, compileGlyphs(item[0]))
+		fmt.Fprintf(stderrWriter(), "\033[2K\r%s %d/%d %s", DimStyle.Render(msg), current, total, compileGlyphs(item[0]))
 	} else {
-		fmt.Fprintf(os.Stderr, "\033[2K\r%s %d/%d", DimStyle.Render(msg), current, total)
+		fmt.Fprintf(stderrWriter(), "\033[2K\r%s %d/%d", DimStyle.Render(msg), current, total)
 	}
 }
 
 // ProgressDone clears the progress line.
 func ProgressDone() {
-	fmt.Fprint(os.Stderr, "\033[2K\r")
+	fmt.Fprint(stderrWriter(), "\033[2K\r")
 }
 
 // Label prints a "Label: value" line.
 func Label(word, value string) {
-	fmt.Printf("%s %s\n", KeyStyle.Render(compileGlyphs(i18n.Label(word))), compileGlyphs(value))
+	fmt.Fprintf(stdoutWriter(), "%s %s\n", KeyStyle.Render(compileGlyphs(i18n.Label(word))), compileGlyphs(value))
 }
 
 // Scanln reads from stdin.
@@ -140,7 +139,7 @@ func Scanln(a ...any) (int, error) {
 //	cli.Task("php", "Running tests...")  // [php] Running tests...
 //	cli.Task("go", i18n.Progress("build"))  // [go] Building...
 func Task(label, message string) {
-	fmt.Printf("%s %s\n\n", DimStyle.Render("["+compileGlyphs(label)+"]"), compileGlyphs(message))
+	fmt.Fprintf(stdoutWriter(), "%s %s\n\n", DimStyle.Render("["+compileGlyphs(label)+"]"), compileGlyphs(message))
 }
 
 // Section prints a section header: "── SECTION ──"
@@ -149,7 +148,7 @@ func Task(label, message string) {
 func Section(name string) {
 	dash := Glyph(":dash:")
 	header := dash + dash + " " + strings.ToUpper(compileGlyphs(name)) + " " + dash + dash
-	fmt.Println(AccentStyle.Render(header))
+	fmt.Fprintln(stdoutWriter(), AccentStyle.Render(header))
 }
 
 // Hint prints a labelled hint: "label: message"
@@ -157,7 +156,7 @@ func Section(name string) {
 //	cli.Hint("install", "composer require vimeo/psalm")
 //	cli.Hint("fix", "core php fmt --fix")
 func Hint(label, message string) {
-	fmt.Printf("  %s %s\n", DimStyle.Render(compileGlyphs(label)+":"), compileGlyphs(message))
+	fmt.Fprintf(stdoutWriter(), "  %s %s\n", DimStyle.Render(compileGlyphs(label)+":"), compileGlyphs(message))
 }
 
 // Severity prints a severity-styled message.
@@ -180,7 +179,7 @@ func Severity(level, message string) {
 	default:
 		style = DimStyle
 	}
-	fmt.Printf("  %s %s\n", style.Render("["+compileGlyphs(level)+"]"), compileGlyphs(message))
+	fmt.Fprintf(stdoutWriter(), "  %s %s\n", style.Render("["+compileGlyphs(level)+"]"), compileGlyphs(message))
 }
 
 // Result prints a result line: "✓ message" or "✗ message"

pkg/cli/output_test.go

@@ -159,3 +159,27 @@ func TestScanln_UsesOverrideStdin(t *testing.T) {
 		t.Fatalf("expected %q, got %q", "hello", got)
 	}
 }
+
+func TestOutputSetters_Good(t *testing.T) {
+	var out bytes.Buffer
+	var err bytes.Buffer
+
+	SetStdout(&out)
+	SetStderr(&err)
+	t.Cleanup(func() {
+		SetStdout(nil)
+		SetStderr(nil)
+	})
+
+	Success("done")
+	Error("fail")
+	Info("note")
+	Warn("careful")
+
+	if out.Len() == 0 {
+		t.Fatal("expected stdout writer to receive output")
+	}
+	if err.Len() == 0 {
+		t.Fatal("expected stderr writer to receive output")
+	}
+}

pkg/cli/prompt.go

@@ -5,29 +5,16 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"os"
 	"strconv"
 	"strings"
 )
 
-var stdin io.Reader = os.Stdin
-
-// SetStdin overrides the default stdin reader for testing.
-// Pass nil to restore the real os.Stdin reader.
-func SetStdin(r io.Reader) {
-	if r == nil {
-		stdin = os.Stdin
-		return
-	}
-	stdin = r
-}
-
 // newReader wraps stdin in a bufio.Reader if it isn't one already.
 func newReader() *bufio.Reader {
-	if br, ok := stdin.(*bufio.Reader); ok {
+	if br, ok := stdinReader().(*bufio.Reader); ok {
 		return br
 	}
-	return bufio.NewReader(stdin)
+	return bufio.NewReader(stdinReader())
 }
 
 // Prompt asks for text input with a default value.
@@ -35,9 +22,9 @@ func Prompt(label, defaultVal string) (string, error) {
 	label = compileGlyphs(label)
 	defaultVal = compileGlyphs(defaultVal)
 	if defaultVal != "" {
-		fmt.Fprintf(os.Stderr, "%s [%s]: ", label, defaultVal)
+		fmt.Fprintf(stderrWriter(), "%s [%s]: ", label, defaultVal)
 	} else {
-		fmt.Fprintf(os.Stderr, "%s: ", label)
+		fmt.Fprintf(stderrWriter(), "%s: ", label)
 	}
 
 	r := newReader()
@@ -66,11 +53,11 @@ func Select(label string, options []string) (string, error) {
 		return "", nil
 	}
 
-	fmt.Fprintln(os.Stderr, compileGlyphs(label))
+	fmt.Fprintln(stderrWriter(), compileGlyphs(label))
 	for i, opt := range options {
-		fmt.Fprintf(os.Stderr, "  %d. %s\n", i+1, compileGlyphs(opt))
+		fmt.Fprintf(stderrWriter(), "  %d. %s\n", i+1, compileGlyphs(opt))
 	}
-	fmt.Fprintf(os.Stderr, "Choose [1-%d]: ", len(options))
+	fmt.Fprintf(stderrWriter(), "Choose [1-%d]: ", len(options))
 
 	r := newReader()
 	input, err := r.ReadString('\n')
@@ -94,11 +81,11 @@ func MultiSelect(label string, options []string) ([]string, error) {
 		return []string{}, nil
 	}
 
-	fmt.Fprintln(os.Stderr, compileGlyphs(label))
+	fmt.Fprintln(stderrWriter(), compileGlyphs(label))
 	for i, opt := range options {
-		fmt.Fprintf(os.Stderr, "  %d. %s\n", i+1, compileGlyphs(opt))
+		fmt.Fprintf(stderrWriter(), "  %d. %s\n", i+1, compileGlyphs(opt))
 	}
-	fmt.Fprintf(os.Stderr, "Choose (space-separated) [1-%d]: ", len(options))
+	fmt.Fprintf(stderrWriter(), "Choose (space-separated) [1-%d]: ", len(options))
 
 	r := newReader()
 	input, err := r.ReadString('\n')

pkg/cli/prompt_test.go

@@ -395,6 +395,20 @@ func TestSetStdin_Good_ResetNil(t *testing.T) {
 	assert.Same(t, os.Stdin, stdin)
 }
 
+func TestPrompt_Good_UsesStderrSetter(t *testing.T) {
+	SetStdin(strings.NewReader("alice\n"))
+	defer SetStdin(nil)
+
+	var errBuf bytes.Buffer
+	SetStderr(&errBuf)
+	defer SetStderr(nil)
+
+	val, err := Prompt("Name", "")
+	assert.NoError(t, err)
+	assert.Equal(t, "alice", val)
+	assert.Contains(t, errBuf.String(), "Name")
+}
+
 func TestPromptHints_Good_UseStderr(t *testing.T) {
 	oldOut := os.Stdout
 	oldErr := os.Stderr

pkg/cli/render.go

@@ -41,7 +41,7 @@ func UseRenderBoxed() { currentRenderStyle = RenderBoxed }
 
 // Render outputs the layout to terminal.
 func (c *Composite) Render() {
-	fmt.Print(c.String())
+	fmt.Fprint(stdoutWriter(), c.String())
 }
 
 // String returns the rendered layout.

pkg/cli/styles.go

@@ -275,7 +275,7 @@ func (t *Table) String() string {
 
 // Render prints the table to stdout.
 func (t *Table) Render() {
-	fmt.Print(t.String())
+	fmt.Fprint(stdoutWriter(), t.String())
 }
 
 func (t *Table) colCount() int {

pkg/cli/tracker.go

@@ -120,7 +120,7 @@ func (tr *TaskTracker) Snapshots() iter.Seq2[string, string] {
 
 // NewTaskTracker creates a new parallel task tracker.
 func NewTaskTracker() *TaskTracker {
-	return &TaskTracker{out: os.Stderr}
+	return &TaskTracker{out: stderrWriter()}
 }
 
 // WithOutput sets the destination writer for tracker output.

pkg/cli/tree.go

@@ -79,7 +79,7 @@ func (n *TreeNode) String() string {
 
 // Render prints the tree to stdout.
 func (n *TreeNode) Render() {
-	fmt.Print(n.String())
+	fmt.Fprint(stdoutWriter(), n.String())
 }
 
 func (n *TreeNode) renderLabel() string {

pkg/cli/utils.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"os"
 	"os/exec"
 	"strings"
 	"time"
@@ -44,11 +43,11 @@ type confirmConfig struct {
 }
 
 func promptHint(msg string) {
-	fmt.Fprintln(os.Stderr, DimStyle.Render(compileGlyphs(msg)))
+	fmt.Fprintln(stderrWriter(), DimStyle.Render(compileGlyphs(msg)))
 }
 
 func promptWarning(msg string) {
-	fmt.Fprintln(os.Stderr, WarningStyle.Render(compileGlyphs(msg)))
+	fmt.Fprintln(stderrWriter(), WarningStyle.Render(compileGlyphs(msg)))
 }
 
 // DefaultYes sets the default response to "yes" (pressing Enter confirms).
@@ -114,7 +113,7 @@ func Confirm(prompt string, opts ...ConfirmOption) bool {
 	reader := newReader()
 
 	for {
-		fmt.Fprintf(os.Stderr, "%s %s", prompt, suffix)
+		fmt.Fprintf(stderrWriter(), "%s %s", prompt, suffix)
 
 		var response string
 		var readErr error
@@ -134,7 +133,7 @@ func Confirm(prompt string, opts ...ConfirmOption) bool {
 				readErr = <-errChan
 				response = strings.ToLower(strings.TrimSpace(response))
 			case <-time.After(cfg.timeout):
-				fmt.Fprintln(os.Stderr) // New line after timeout
+				fmt.Fprintln(stderrWriter()) // New line after timeout
 				return cfg.defaultYes
 			}
 		} else {
@@ -251,9 +250,9 @@ func Question(prompt string, opts ...QuestionOption) string {
 	for {
 		// Build prompt with default
 		if cfg.defaultValue != "" {
-			fmt.Fprintf(os.Stderr, "%s [%s] ", prompt, compileGlyphs(cfg.defaultValue))
+			fmt.Fprintf(stderrWriter(), "%s [%s] ", prompt, compileGlyphs(cfg.defaultValue))
 		} else {
-			fmt.Fprintf(os.Stderr, "%s ", prompt)
+			fmt.Fprintf(stderrWriter(), "%s ", prompt)
 		}
 
 		response, err := reader.ReadString('\n')
@@ -375,9 +374,9 @@ func Choose[T any](prompt string, items []T, opts ...ChooseOption[T]) T {
 		renderChoices(prompt, items, visible, cfg.displayFn, cfg.defaultN, cfg.filter)
 
 		if cfg.filter {
-			fmt.Fprintf(os.Stderr, "Enter number [1-%d] or filter: ", len(visible))
+			fmt.Fprintf(stderrWriter(), "Enter number [1-%d] or filter: ", len(visible))
 		} else {
-			fmt.Fprintf(os.Stderr, "Enter number [1-%d]: ", len(visible))
+			fmt.Fprintf(stderrWriter(), "Enter number [1-%d]: ", len(visible))
 		}
 		response, err := reader.ReadString('\n')
 		response = strings.TrimSpace(response)
@@ -474,9 +473,9 @@ func ChooseMulti[T any](prompt string, items []T, opts ...ChooseOption[T]) []T {
 		renderChoices(prompt, items, visible, cfg.displayFn, -1, cfg.filter)
 
 		if cfg.filter {
-			fmt.Fprint(os.Stderr, "Enter numbers (e.g., 1 3 5 or 1-3), or filter text, or empty for none: ")
+			fmt.Fprint(stderrWriter(), "Enter numbers (e.g., 1 3 5 or 1-3), or filter text, or empty for none: ")
 		} else {
-			fmt.Fprint(os.Stderr, "Enter numbers (e.g., 1 3 5 or 1-3) or empty for none: ")
+			fmt.Fprint(stderrWriter(), "Enter numbers (e.g., 1 3 5 or 1-3) or empty for none: ")
 		}
 		response, _ := reader.ReadString('\n')
 		response = strings.TrimSpace(response)
@@ -512,16 +511,16 @@ func ChooseMulti[T any](prompt string, items []T, opts ...ChooseOption[T]) []T {
 }
 
 func renderChoices[T any](prompt string, items []T, visible []int, displayFn func(T) string, defaultN int, filter bool) {
-	fmt.Fprintln(os.Stderr, prompt)
+	fmt.Fprintln(stderrWriter(), prompt)
 	for i, idx := range visible {
 		marker := " "
 		if defaultN >= 0 && idx == defaultN {
 			marker = "*"
 		}
-		fmt.Fprintf(os.Stderr, "  %s%d. %s\n", marker, i+1, compileGlyphs(displayFn(items[idx])))
+		fmt.Fprintf(stderrWriter(), "  %s%d. %s\n", marker, i+1, compileGlyphs(displayFn(items[idx])))
 	}
 	if filter {
-		fmt.Fprintln(os.Stderr, "  (type to filter the list)")
+		fmt.Fprintln(stderrWriter(), "  (type to filter the list)")
 	}
 }