diff --git a/cmd/ncdu/scan/scan_test.go b/cmd/ncdu/scan/scan_test.go
new file mode 100644
index 000000000..b66807eaf
--- /dev/null
+++ b/cmd/ncdu/scan/scan_test.go
@@ -0,0 +1,247 @@
+package scan
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	_ "github.com/rclone/rclone/backend/local"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fstest"
+	"github.com/rclone/rclone/fstest/mockdir"
+	"github.com/rclone/rclone/fstest/mockobject"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestScan(t *testing.T) {
+	fstest.Initialise()
+
+	// testfiles (from cmd/tree) contains: file1, file2, file3 (empty), subdir/{file4,file5} (empty)
+	f, err := fs.NewFs(context.Background(), "../../tree/testfiles")
+	require.NoError(t, err)
+
+	rootChan, errChan, updatedChan := Scan(context.Background(), f)
+
+	root := <-rootChan
+	require.NotNil(t, root)
+	require.NoError(t, <-errChan)
+
+	assert.Equal(t, "", root.Path())
+	assert.Nil(t, root.Parent())
+
+	size, count := root.Attr()
+	assert.Equal(t, int64(0), size)  // all files are empty
+	assert.Equal(t, int64(5), count) // 3 root files + 2 subdir files
+
+	// Entries are sorted: file1, file2, file3, subdir
+	require.Len(t, root.Entries(), 4)
+	i := indexByName(root.Entries(), "subdir")
+	require.NotEqual(t, -1, i, "subdir not found")
+	subDir, _ := root.GetDir(i)
+	require.NotNil(t, subDir)
+
+	assert.Equal(t, "subdir", subDir.Path())
+	assert.Equal(t, root, subDir.Parent())
+
+	subSize, subCount := subDir.Attr()
+	assert.Equal(t, int64(0), subSize)
+	assert.Equal(t, int64(2), subCount)
+
+	select {
+	case <-updatedChan:
+		// at least one update was signalled during the scan
+	default:
+		t.Error("expected at least one update signal from Scan")
+	}
+}
+
+// indexByName returns the index of the entry with the given name, or -1 if not found.
+func indexByName(entries fs.DirEntries, name string) int {
+	for i, e := range entries {
+		if e.Remote() == name {
+			return i
+		}
+	}
+	return -1
+}
+
+func TestAttrsAverageSize(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		a    Attrs
+		want float64
+	}{
+		{"no files", Attrs{Count: 0}, 0},
+		{"all sizes known", Attrs{Count: 4, Size: 100}, 25},
+		{"some sizes unknown", Attrs{Count: 4, CountUnknownSize: 2, Size: 100}, 50},
+		{"all sizes unknown", Attrs{Count: 3, CountUnknownSize: 3}, 0},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.want, tc.a.AverageSize())
+		})
+	}
+}
+
+// fileEntry returns a mock fs.Object with the given name and content.
+// The size of the object equals len(content).
+func fileEntry(name, content string) fs.Object {
+	return mockobject.New(name).WithContent([]byte(content), mockobject.SeekModeNone)
+}
+
+func TestNewDirSizeAndCount(t *testing.T) {
+	entries := fs.DirEntries{
+		fileEntry("file1", "hello"), // 5 bytes
+		fileEntry("file2", "hi"),    // 2 bytes
+		mockdir.New("subdir"),
+	}
+	d := newDir(nil, "", entries, nil)
+
+	size, count := d.Attr()
+	assert.Equal(t, int64(7), size)
+	assert.Equal(t, int64(2), count) // dirs are not counted, only files
+}
+
+func TestNewDirUnknownSize(t *testing.T) {
+	obj := mockobject.New("unknown").WithContent([]byte("some content"), mockobject.SeekModeNone)
+	obj.SetUnknownSize(true) // simulate a backend that doesn't know the object size
+
+	d := newDir(nil, "", fs.DirEntries{obj}, nil)
+
+	size, count := d.Attr()
+	assert.Equal(t, int64(0), size)  // unknown size treated as 0
+	assert.Equal(t, int64(1), count) // still counted as a file
+}
+
+func TestNewDirReadErrorSetsEntriesHaveErrors(t *testing.T) {
+	// AttrI returns subDir.entriesHaveErrors (not subDir.readError directly).
+	// entriesHaveErrors is set on a dir when one of *its* children has a readError.
+	// So we need three levels: root → child → grandchild(readError),
+	// then root.AttrI(child) reflects child.entriesHaveErrors == true.
+	root := newDir(nil, "", fs.DirEntries{mockdir.New("child")}, nil)
+	child := newDir(root, "child", fs.DirEntries{mockdir.New("child/grand")}, nil)
+	newDir(child, "child/grand", fs.DirEntries{}, fmt.Errorf("permission denied"))
+
+	i := indexByName(root.Entries(), "child")
+	require.NotEqual(t, -1, i)
+	attrs, err := root.AttrI(i)
+	assert.NoError(t, err)
+	assert.True(t, attrs.EntriesHaveErrors)
+}
+
+func TestAttrIFile(t *testing.T) {
+	d := newDir(nil, "", fs.DirEntries{fileEntry("f", "hello")}, nil)
+	attrs, err := d.AttrI(0)
+	assert.NoError(t, err)
+	assert.False(t, attrs.IsDir)
+	assert.True(t, attrs.Readable)
+	assert.Equal(t, int64(5), attrs.Size)
+}
+
+func TestAttrIDirLoaded(t *testing.T) {
+	root := newDir(nil, "", fs.DirEntries{mockdir.New("child")}, nil)
+	newDir(root, "child", fs.DirEntries{fileEntry("child/f", "hello")}, nil)
+
+	i := indexByName(root.Entries(), "child")
+	require.NotEqual(t, -1, i, "child not found")
+	attrs, err := root.AttrI(i)
+	assert.NoError(t, err)
+	assert.True(t, attrs.IsDir)
+	assert.True(t, attrs.Readable)
+	assert.Equal(t, int64(5), attrs.Size)
+	assert.Equal(t, int64(1), attrs.Count)
+}
+
+func TestAttrIDirUnloaded(t *testing.T) {
+	// Directory entry present in entries but no child Dir created yet.
+	d := newDir(nil, "", fs.DirEntries{mockdir.New("missing")}, nil)
+	attrs, err := d.AttrI(0)
+	assert.NoError(t, err)
+	assert.True(t, attrs.IsDir)
+	assert.False(t, attrs.Readable)
+	assert.Equal(t, int64(0), attrs.Size)
+}
+
+func TestAttrWithModTimeI(t *testing.T) {
+	obj := mockobject.New("file").WithContent([]byte("abc"), mockobject.SeekModeNone)
+	d := newDir(nil, "", fs.DirEntries{obj}, nil)
+	attrs, err := d.AttrWithModTimeI(context.Background(), 0)
+	assert.NoError(t, err)
+	assert.False(t, attrs.IsDir)
+	assert.Equal(t, int64(3), attrs.Size)
+	assert.Equal(t, obj.ModTime(context.Background()), attrs.ModTime)
+}
+
+func TestRemoveFileUpdatesDir(t *testing.T) {
+	entries := fs.DirEntries{
+		fileEntry("file1", "hello"), // 5 bytes
+		fileEntry("file2", "hi"),    // 2 bytes
+	}
+	d := newDir(nil, "", entries, nil)
+
+	i := indexByName(d.Entries(), "file1")
+	require.NotEqual(t, -1, i)
+	d.Remove(i)
+
+	size, count := d.Attr()
+	assert.Equal(t, int64(2), size)
+	assert.Equal(t, int64(1), count)
+	assert.Len(t, d.Entries(), 1)
+}
+
+func TestRemoveUnknownSizeFile(t *testing.T) {
+	obj := mockobject.New("unknown").WithContent([]byte("data"), mockobject.SeekModeNone)
+	obj.SetUnknownSize(true)
+
+	d := newDir(nil, "", fs.DirEntries{obj}, nil)
+	d.Remove(0)
+
+	size, count := d.Attr()
+	assert.Equal(t, int64(0), size)
+	assert.Equal(t, int64(0), count)
+}
+
+func TestRemoveDirUpdatesParent(t *testing.T) {
+	root := newDir(nil, "", fs.DirEntries{mockdir.New("child")}, nil)
+	newDir(root, "child", fs.DirEntries{
+		fileEntry("child/file1", "hello"), // 5 bytes
+		fileEntry("child/file2", "world"), // 5 bytes
+	}, nil)
+
+	rootSize, rootCount := root.Attr()
+	assert.Equal(t, int64(10), rootSize)
+	assert.Equal(t, int64(2), rootCount)
+
+	i := indexByName(root.Entries(), "child")
+	require.NotEqual(t, -1, i)
+	root.Remove(i)
+
+	rootSize, rootCount = root.Attr()
+	assert.Equal(t, int64(0), rootSize)
+	assert.Equal(t, int64(0), rootCount)
+	assert.Empty(t, root.Entries())
+}
+
+func TestRemoveFilePropagatesCountsToParent(t *testing.T) {
+	root := newDir(nil, "", fs.DirEntries{mockdir.New("child")}, nil)
+
+	childEntries := fs.DirEntries{
+		fileEntry("child/file1", "hello"), // 5 bytes
+		fileEntry("child/file2", "world"), // 5 bytes
+	}
+	child := newDir(root, "child", childEntries, nil)
+
+	// After building the tree: root accumulates child's files (10 bytes, 2 files)
+	rootSize, rootCount := root.Attr()
+	assert.Equal(t, int64(10), rootSize)
+	assert.Equal(t, int64(2), rootCount)
+
+	i := indexByName(child.Entries(), "child/file1")
+	require.NotEqual(t, -1, i)
+	child.Remove(i)
+
+	// After removing child/file1: root should reflect the remaining 5 bytes and 1 file
+	rootSize, rootCount = root.Attr()
+	assert.Equal(t, int64(5), rootSize)
+	assert.Equal(t, int64(1), rootCount)
+}