gui: History and undo tab; each plan and undo is its own run

7 files changed, 985 insertions, 54 deletions

diff --git a/gui/internal/model/history.go b/gui/internal/model/history.go
new file mode 100644
index 0000000..9ce854c
--- /dev/null
+++ b/gui/internal/model/history.go
@@ -0,0 +1,280 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package model
+
+import (
+	"context"
+	"fmt"
+	"sort"
+	"strings"
+	"time"
+
+	"krino/internal/engine"
+	"krino/internal/journal"
+	"krino/internal/lock"
+)
+
+// Run is one line of the History tab's run list: what a run did, and
+// whether it has since been reversed (GUI design §4).
+type Run struct {
+	ID      string
+	Start   time.Time
+	Dirs    []string
+	Summary string // "3 moved · 1 trashed", or "nothing applied"
+	Note    string // "(undone)", "(partly undone)", or ""
+	UndoOf  string // for an undo run, the run it reverses
+}
+
+// Runs lists the most recent runs, newest first. It reads the log only.
+func Runs(e *engine.Engine, n int) ([]Run, error) {
+	rs, err := journal.Runs(e.Config.LogFile(), n)
+	if err != nil {
+		return nil, err
+	}
+	out := make([]Run, 0, len(rs))
+	for _, r := range rs {
+		out = append(out, Run{
+			ID:      r.ID,
+			Start:   r.Start,
+			Dirs:    r.Dirs,
+			Summary: countsText(r.Counts),
+			Note:    note(r),
+			UndoOf:  r.UndoOf,
+		})
+	}
+	return out, nil
+}
+
+// note is what the list says about a run that has been reversed.
+func note(r journal.Run) string {
+	switch {
+	case r.PartlyUndone:
+		return "(partly undone)"
+	case r.Undone:
+		return "(undone)"
+	}
+	return ""
+}
+
+// pastTense and countOrder are krino log's own display words and order
+// (cmd/krino/log.go): the log's action names are the wire format and are
+// never renamed, so both front ends translate them the same way.
+var pastTense = map[string]string{
+	"copy": "copied", "move": "moved", "rename": "renamed",
+	"trash": "trashed", "delete": "deleted", "displace": "displaced",
+	"undo-copy": "undo-copied", "undo-move": "undo-moved",
+	"undo-rename": "undo-renamed", "undo-trash": "undo-trashed",
+	"undo-displace": "undo-displaced",
+}
+
+var countOrder = []string{
+	"copy", "move", "rename", "trash", "delete", "displace",
+	"undo-copy", "undo-move", "undo-rename", "undo-trash", "undo-displace",
+}
+
+// countsText renders a run's counts in a fixed order, skipping actions with
+// no successful entries.
+func countsText(counts map[string]int) string {
+	var parts []string
+	for _, action := range countOrder {
+		if n := counts[action]; n > 0 {
+			parts = append(parts, fmt.Sprintf("%d %s", n, pastTense[action]))
+		}
+	}
+	if len(parts) == 0 {
+		return "nothing applied"
+	}
+	return strings.Join(parts, " · ")
+}
+
+// UndoRow is one file of a run's reversal.
+type UndoRow struct {
+	File    string // the Rel the original run logged
+	Dir     string
+	Steps   []engine.UndoStep
+	Refused string // non-empty: nothing here is reversed, and why
+	// Selected is the checkbox. A refused file has none.
+	Selected bool
+	Actable  bool
+	Outcome  string // "", "done", "failed: ...", "declined"
+}
+
+// UndoCounts is the header "N files · N to reverse · N refused".
+type UndoCounts struct {
+	Files, ToReverse, Refused int
+}
+
+// UndoTab is one run's reversal, ready to show and approve.
+type UndoTab struct {
+	Run     string
+	Rows    []UndoRow
+	Counts  UndoCounts
+	Applied bool
+
+	sess  *engine.Session
+	up    *engine.UndoPlan
+	locks []*lock.Lock
+}
+
+// PlanUndo builds the reversal of runID and locks every directory it
+// touches, in name order, so nothing moves under the user while they
+// choose; a directory another krino holds stops the whole undo, named
+// (spec §10, GUI design §4). Nothing is touched until Apply.
+//
+// Like a plan, an undo gets its own session: reversing a run is a run of
+// its own, logged under a new id that says which run it undoes.
+func PlanUndo(ctx context.Context, e *engine.Engine, runID string) (*UndoTab, error) {
+	sess, err := e.NewSession(false)
+	if err != nil {
+		return nil, err
+	}
+	up, err := sess.PlanUndo(runID)
+	if err != nil {
+		sess.Close()
+		return nil, err
+	}
+	locks, err := sess.LockDirs(ctx, undoDirNames(up.Files), false)
+	if err != nil {
+		sess.Close()
+		return nil, err
+	}
+	t := &UndoTab{Run: up.Run, sess: sess, up: up, locks: locks}
+	t.fill()
+	return t, nil
+}
+
+// fill turns the engine's undo plan into rows and counts.
+func (t *UndoTab) fill() {
+	t.Rows = nil
+	t.Counts = UndoCounts{Files: len(t.up.Files)}
+	for _, f := range t.up.Files {
+		row := UndoRow{File: f.File, Dir: f.Dir, Steps: f.Steps, Refused: f.Refused}
+		row.Actable = f.Refused == ""
+		row.Selected = row.Actable
+		if row.Actable {
+			t.Counts.ToReverse++
+		} else {
+			t.Counts.Refused++
+		}
+		t.Rows = append(t.Rows, row)
+	}
+}
+
+// undoDirNames is every directory the plan touches, once, in name order.
+func undoDirNames(files []engine.UndoFile) []string {
+	seen := map[string]bool{}
+	var out []string
+	for _, f := range files {
+		if f.Dir != "" && !seen[f.Dir] {
+			seen[f.Dir] = true
+			out = append(out, f.Dir)
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
+// SelectAll checks every file that can be reversed; SelectNone clears them.
+func (t *UndoTab) SelectAll()  { t.setAll(true) }
+func (t *UndoTab) SelectNone() { t.setAll(false) }
+
+func (t *UndoTab) setAll(on bool) {
+	for i := range t.Rows {
+		t.Rows[i].Selected = on && t.Rows[i].Actable
+	}
+}
+
+// Toggle flips row i's checkbox; a refused row stays off.
+func (t *UndoTab) Toggle(i int) {
+	if i < 0 || i >= len(t.Rows) || !t.Rows[i].Actable {
+		return
+	}
+	t.Rows[i].Selected = !t.Rows[i].Selected
+}
+
+// SelectedCount is how many files Undo would reverse.
+func (t *UndoTab) SelectedCount() int {
+	n := 0
+	for _, r := range t.Rows {
+		if r.Selected {
+			n++
+		}
+	}
+	return n
+}
+
+// Apply reverses the checked files. An unchecked one is not dropped: it is
+// logged as declined, exactly as the terminal review logs a file the user
+// said no to (spec §9). Refused files ride along unchanged, as they do on
+// the command line. The locks are released afterwards: the plan is history.
+func (t *UndoTab) Apply(ctx context.Context) (*engine.ApplyResult, error) {
+	toApply := &engine.UndoPlan{Run: t.up.Run, Cleanup: t.up.Cleanup}
+	for i, f := range t.up.Files {
+		if f.Refused == "" && !t.Rows[i].Selected {
+			f.Declined = true
+		}
+		toApply.Files = append(toApply.Files, f)
+	}
+	res, err := t.sess.ApplyUndo(ctx, toApply)
+	t.Applied = true
+	t.Close()
+	if res != nil {
+		t.record(res)
+	}
+	return res, err
+}
+
+// record writes each file's outcome onto its row, matching on the file name
+// the undo result carries.
+func (t *UndoTab) record(res *engine.ApplyResult) {
+	rows := map[string]int{}
+	for i, r := range t.Rows {
+		rows[r.File] = i
+	}
+	for _, fr := range res.Files {
+		if i, ok := rows[fr.File.Rel]; ok {
+			t.Rows[i].Outcome = undoOutcome(t.Rows[i], fr)
+		}
+	}
+}
+
+// undoOutcome is what happened to one file. Removing a directory the
+// original run made is tidiness, not a restoration - it fails whenever
+// something else still lives there, a declined file of this very undo
+// included - so a failed undo-mkdir does not make the file itself a
+// failure, exactly as the engine's own counts treat it.
+func undoOutcome(row UndoRow, fr engine.FileResult) string {
+	out := "done"
+	for i, sr := range fr.Steps {
+		action := ""
+		if i < len(row.Steps) {
+			action = row.Steps[i].Action
+		}
+		switch sr.Status {
+		case "failed":
+			if action != "undo-mkdir" {
+				return "failed: " + sr.Detail
+			}
+		case "declined":
+			out = "declined"
+		}
+	}
+	return out
+}
+
+// Close releases the directories' locks and ends the run, which Apply also
+// does. One lock's failure never stops the rest from being released, or the
+// session from being closed. Closing twice is not an error.
+func (t *UndoTab) Close() error {
+	var first error
+	for _, l := range t.locks {
+		if err := l.Release(); err != nil && first == nil {
+			first = err
+		}
+	}
+	t.locks = nil
+	if err := t.sess.Close(); err != nil && first == nil {
+		first = err
+	}
+	return first
+}
diff --git a/gui/internal/model/history_test.go b/gui/internal/model/history_test.go
new file mode 100644
index 0000000..a486c2e
--- /dev/null
+++ b/gui/internal/model/history_test.go
@@ -0,0 +1,207 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package model
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"krino/internal/engine"
+	"krino/internal/lock"
+)
+
+// applyOneRun sorts the sandbox directory and returns the run id it wrote,
+// so the history tests have something to look back at.
+func applyOneRun(t *testing.T, e *engine.Engine) string {
+	t.Helper()
+	tab, err := Plan(context.Background(), e, e.Dirs[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := tab.Apply(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	return tab.Run()
+}
+
+// TestRunsListsWhatHappened: the run list shows the run just applied, with
+// its directory and its counts in krino log's words, and says so once the
+// run has been undone.
+func TestRunsListsWhatHappened(t *testing.T) {
+	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
+	e, _ := sandboxDir(t, conf, map[string]string{"a.pdf": "one", "b.pdf": "two"})
+	id := applyOneRun(t, e)
+
+	runs, err := Runs(e, 50)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(runs) != 1 {
+		t.Fatalf("runs = %+v, want the one run", runs)
+	}
+	r := runs[0]
+	if r.ID != id || len(r.Dirs) != 1 || r.Dirs[0] != "dl" {
+		t.Errorf("run = %+v, want %s in dl", r, id)
+	}
+	if r.Summary != "2 moved" {
+		t.Errorf("summary = %q, want %q", r.Summary, "2 moved")
+	}
+	if r.Note != "" || r.UndoOf != "" {
+		t.Errorf("a fresh run is not marked: %+v", r)
+	}
+
+	// Undo it, and the list says so - and carries the undo run itself.
+	tab, err := PlanUndo(context.Background(), e, id)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := tab.Apply(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	tab.Close()
+	runs, err = Runs(e, 50)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(runs) != 2 {
+		t.Fatalf("runs = %+v, want the run and its undo", runs)
+	}
+	if runs[0].UndoOf != id {
+		t.Errorf("newest run = %+v, want the undo of %s", runs[0], id)
+	}
+	if runs[1].Note != "(undone)" {
+		t.Errorf("undone run = %+v, want (undone)", runs[1])
+	}
+}
+
+// TestUndoRowsAndSelection: every file of the run is a row, unchecked files
+// are logged as declined rather than reversed, and the counts read as the
+// terminal's header does.
+func TestUndoRowsAndSelection(t *testing.T) {
+	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
+	e, h := sandboxDir(t, conf, map[string]string{"a.pdf": "one", "b.pdf": "two"})
+	id := applyOneRun(t, e)
+
+	tab, err := PlanUndo(context.Background(), e, id)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer tab.Close()
+	if tab.Counts.Files != 2 || tab.Counts.ToReverse != 2 || tab.Counts.Refused != 0 {
+		t.Fatalf("counts = %+v", tab.Counts)
+	}
+	if tab.SelectedCount() != 2 {
+		t.Fatalf("rows do not start selected: %+v", tab.Rows)
+	}
+	for i, r := range tab.Rows {
+		if r.File == "b.pdf" {
+			tab.Toggle(i)
+		}
+	}
+	res, err := tab.Apply(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Applied != 1 || res.Declined != 1 {
+		t.Errorf("result = %+v", res)
+	}
+	if _, err := os.Stat(filepath.Join(h, "dl", "a.pdf")); err != nil {
+		t.Errorf("the checked file was not put back: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(h, "dl", "Out", "b.pdf")); err != nil {
+		t.Errorf("the unchecked file was put back anyway: %v", err)
+	}
+	for _, r := range tab.Rows {
+		want := "done"
+		if r.File == "b.pdf" {
+			want = "declined"
+		}
+		if r.Outcome != want {
+			t.Errorf("%s: outcome %q, want %q", r.File, r.Outcome, want)
+		}
+	}
+}
+
+// TestUndoRefusedRowIsNotSelectable: a file undo will not touch - here its
+// destination is gone - is shown with the reason and can never be checked
+// (GUI design §4).
+func TestUndoRefusedRowIsNotSelectable(t *testing.T) {
+	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
+	e, h := sandboxDir(t, conf, map[string]string{"a.pdf": "one", "b.pdf": "two"})
+	id := applyOneRun(t, e)
+	if err := os.Remove(filepath.Join(h, "dl", "Out", "a.pdf")); err != nil {
+		t.Fatal(err)
+	}
+
+	tab, err := PlanUndo(context.Background(), e, id)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer tab.Close()
+	if tab.Counts.Refused != 1 || tab.Counts.ToReverse != 1 {
+		t.Fatalf("counts = %+v", tab.Counts)
+	}
+	tab.SelectAll()
+	for i, r := range tab.Rows {
+		if r.File != "a.pdf" {
+			continue
+		}
+		if r.Refused == "" {
+			t.Errorf("the missing file carries no reason: %+v", r)
+		}
+		if r.Selected || r.Actable {
+			t.Errorf("a refused row is selectable: %+v", r)
+		}
+		tab.Toggle(i)
+		if tab.Rows[i].Selected {
+			t.Error("Toggle checked a refused row")
+		}
+	}
+}
+
+// TestUndoHoldsTheDirectoryLocks: while an undo plan is open nothing else
+// may work in the directories it covers, and Close - like Apply - frees
+// them (GUI design §4).
+func TestUndoHoldsTheDirectoryLocks(t *testing.T) {
+	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
+	e, _ := sandboxDir(t, conf, map[string]string{"a.pdf": "one"})
+	id := applyOneRun(t, e)
+
+	tab, err := PlanUndo(context.Background(), e, id)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := lock.Acquire(context.Background(), e.Config.LockFile("dl"), false); !errors.Is(err, lock.ErrHeld) {
+		t.Fatalf("an open undo plan does not hold the lock: %v", err)
+	}
+	if err := tab.Close(); err != nil {
+		t.Fatal(err)
+	}
+	l, err := lock.Acquire(context.Background(), e.Config.LockFile("dl"), false)
+	if err != nil {
+		t.Fatalf("closing the tab did not release the lock: %v", err)
+	}
+	l.Release()
+}
+
+// TestPlanUndoRefusesAHeldDirectory: a directory another krino is working
+// in stops the undo before anything is shown, and the message names it.
+func TestPlanUndoRefusesAHeldDirectory(t *testing.T) {
+	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
+	e, _ := sandboxDir(t, conf, map[string]string{"a.pdf": "one"})
+	id := applyOneRun(t, e)
+	held, err := lock.Acquire(context.Background(), e.Config.LockFile("dl"), false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer held.Release()
+	if _, err := PlanUndo(context.Background(), e, id); !errors.Is(err, lock.ErrHeld) {
+		t.Fatalf("PlanUndo = %v, want lock.ErrHeld", err)
+	} else if !strings.Contains(err.Error(), "dl") {
+		t.Errorf("the message does not name the directory: %v", err)
+	}
+}
diff --git a/gui/internal/model/plan.go b/gui/internal/model/plan.go
index 4066eae..86acab7 100644
--- a/gui/internal/model/plan.go
+++ b/gui/internal/model/plan.go
@@ -54,18 +54,26 @@ type Counts struct {
 // chooses, and no other krino moves the files under them (GUI design §3).
 // A directory another run is already working in is not planned at all: the
 // error is lock.ErrHeld, naming the directory.
-func Plan(ctx context.Context, sess *engine.Session, d *engine.Dir) (*PlanTab, error) {
+//
+// Each plan gets its own session, so applying it is one run of krino with
+// its own run id in the log - the same shape a command line invocation
+// writes. A window is not a run: one that sorted a directory and later
+// undid something must not log both under one id, which would make the run
+// the undo of itself.
+func Plan(ctx context.Context, e *engine.Engine, d *engine.Dir) (*PlanTab, error) {
+	sess, err := e.NewSession(false)
+	if err != nil {
+		return nil, err
+	}
 	l, err := sess.Lock(ctx, d, false)
 	if err != nil {
+		sess.Close()
 		return nil, fmt.Errorf("%s: %w", d.Name, err)
 	}
-	// Start from what this run has actually landed: a name a plan the user
-	// closed had reserved is free again, so looking twice never creeps up
-	// through name-1, name-2 (spec §7.4).
-	sess.FinishDirectory()
 	dp, err := sess.Plan(ctx, d)
 	if err != nil {
 		l.Release()
+		sess.Close()
 		return nil, err
 	}
 	t := &PlanTab{Dir: d, sess: sess, dp: dp, lock: l}
@@ -73,17 +81,22 @@ func Plan(ctx context.Context, sess *engine.Session, d *engine.Dir) (*PlanTab, e
 	return t, nil
 }
 
-// Close releases the directory's lock, which Apply also does once the plan
-// is history. The session outlives the tab, so closing one plan to open
-// another keeps the run - and its claims - going. Closing twice is not an
-// error.
+// Run is the run id this plan was applied under, "" until Apply.
+func (t *PlanTab) Run() string { return t.sess.Run() }
+
+// Close releases the directory's lock and ends the run, which Apply also
+// does once the plan is history. Closing twice is not an error.
 func (t *PlanTab) Close() error {
 	if t.lock == nil {
 		return nil
 	}
 	l := t.lock
 	t.lock = nil
-	return l.Release()
+	err := l.Release()
+	if cerr := t.sess.Close(); err == nil {
+		err = cerr
+	}
+	return err
 }
 
 // fill turns the engine's plan into rows and counts.
@@ -218,12 +231,8 @@ func (t *PlanTab) Apply(ctx context.Context) (*engine.ApplyResult, error) {
 	}
 	res, err := t.sess.Apply(ctx, t.dp, approved)
 	t.Applied = true
-	// The disk is now the truth: a destination this plan reserved but never
-	// used is free again, while one it did use stays protected for the rest
-	// of the run (spec §7.4).
-	t.sess.FinishDirectory()
-	// An applied plan is history, so the directory is free again without
-	// closing the window (GUI design §3).
+	// An applied plan is history: the run is over and the directory free
+	// again, without closing the window (GUI design §3).
 	t.Close()
 	if res != nil {
 		t.record(res)
diff --git a/gui/internal/model/plan_test.go b/gui/internal/model/plan_test.go
index bf8f19f..746ddfc 100644
--- a/gui/internal/model/plan_test.go
+++ b/gui/internal/model/plan_test.go
@@ -52,18 +52,14 @@ func sandboxDir(t *testing.T, conf string, files map[string]string) (*engine.Eng
 	return e, h
 }
 
-func planTab(t *testing.T, e *engine.Engine) (*PlanTab, *engine.Session) {
+func planTab(t *testing.T, e *engine.Engine) *PlanTab {
 	t.Helper()
-	s, err := e.NewSession(false)
+	tab, err := Plan(context.Background(), e, e.Dirs[0])
 	if err != nil {
 		t.Fatal(err)
 	}
-	t.Cleanup(func() { s.Close() })
-	tab, err := Plan(context.Background(), s, e.Dirs[0])
-	if err != nil {
-		t.Fatal(err)
-	}
-	return tab, s
+	t.Cleanup(func() { tab.Close() })
+	return tab
 }
 
 // TestPlanRowsAndCounts: the tab shows one row per file with steps, a row
@@ -79,7 +75,7 @@ func TestPlanRowsAndCounts(t *testing.T) {
 		"c.txt":      strings.Repeat("x", 2048), // over max-read: content unknown
 		"plain.txt":  "nothing",
 	})
-	tab, _ := planTab(t, e)
+	tab := planTab(t, e)
 	if tab.Counts.Scanned != 4 || tab.Counts.Acting != 1 || tab.Counts.Excluded != 1 {
 		t.Errorf("counts = %+v", tab.Counts)
 	}
@@ -111,7 +107,7 @@ func TestPlanRowsAndCounts(t *testing.T) {
 func TestSelectionAndApply(t *testing.T) {
 	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
 	e, h := sandboxDir(t, conf, map[string]string{"a.pdf": "one", "b.pdf": "two"})
-	tab, _ := planTab(t, e)
+	tab := planTab(t, e)
 	if tab.SelectedCount() != 2 {
 		t.Fatalf("rows do not start selected: %+v", tab.Rows)
 	}
@@ -154,7 +150,7 @@ func TestSelectionAndApply(t *testing.T) {
 func TestReplaceWithTrash(t *testing.T) {
 	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
 	e, h := sandboxDir(t, conf, map[string]string{"a.pdf": "one"})
-	tab, _ := planTab(t, e)
+	tab := planTab(t, e)
 	if err := tab.Replace(0, plan.Trash); err != nil {
 		t.Fatal(err)
 	}
@@ -195,7 +191,7 @@ func TestPlanLeavesTheDirectoryAlone(t *testing.T) {
 func TestPlanHoldsTheLock(t *testing.T) {
 	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
 	e, _ := sandboxDir(t, conf, map[string]string{"a.pdf": "one"})
-	tab, _ := planTab(t, e)
+	tab := planTab(t, e)
 	if _, err := lock.Acquire(context.Background(), e.Config.LockFile("dl"), false); !errors.Is(err, lock.ErrHeld) {
 		t.Fatalf("an open plan does not hold the lock: %v", err)
 	}
@@ -217,7 +213,7 @@ func TestPlanHoldsTheLock(t *testing.T) {
 func TestApplyReleasesTheLock(t *testing.T) {
 	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"
 	e, _ := sandboxDir(t, conf, map[string]string{"a.pdf": "one"})
-	tab, _ := planTab(t, e)
+	tab := planTab(t, e)
 	if _, err := tab.Apply(context.Background()); err != nil {
 		t.Fatal(err)
 	}
@@ -241,12 +237,7 @@ func TestPlanRefusesAHeldDirectory(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer held.Release()
-	s, err := e.NewSession(false)
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer s.Close()
-	if _, err := Plan(context.Background(), s, e.Dirs[0]); !errors.Is(err, lock.ErrHeld) {
+	if _, err := Plan(context.Background(), e, e.Dirs[0]); !errors.Is(err, lock.ErrHeld) {
 		t.Fatalf("Plan = %v, want lock.ErrHeld", err)
 	} else if !strings.Contains(err.Error(), "dl") {
 		t.Errorf("the message does not name the directory: %v", err)
@@ -259,7 +250,7 @@ func TestPlanRefusesAHeldDirectory(t *testing.T) {
 func TestRescanForgetsUnusedNames(t *testing.T) {
 	conf := "(path \"~/dl\")\n(rule \"all\" (move \"Out\") (rename \"same.pdf\"))\n"
 	e, _ := sandboxDir(t, conf, map[string]string{"a.pdf": "one", "b.pdf": "two"})
-	tab, s := planTab(t, e)
+	tab := planTab(t, e)
 	// Each file moves into Out and is then renamed; the second file's name
 	// is taken, so the plan reserves a suffixed one for it.
 	want := destinations(t, tab)
@@ -271,7 +262,7 @@ func TestRescanForgetsUnusedNames(t *testing.T) {
 	if err := tab.Close(); err != nil {
 		t.Fatal(err)
 	}
-	again, err := Plan(context.Background(), s, e.Dirs[0])
+	again, err := Plan(context.Background(), e, e.Dirs[0])
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -286,7 +277,7 @@ func TestRescanForgetsUnusedNames(t *testing.T) {
 	if err := again.Close(); err != nil {
 		t.Fatal(err)
 	}
-	third, err := Plan(context.Background(), s, e.Dirs[0])
+	third, err := Plan(context.Background(), e, e.Dirs[0])
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/gui/internal/ui/history.go b/gui/internal/ui/history.go
new file mode 100644
index 0000000..7d4ee7b
--- /dev/null
+++ b/gui/internal/ui/history.go
@@ -0,0 +1,426 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package ui
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/diamondburned/gotk4/pkg/pango"
+
+	"github.com/diamondburned/gotk4/pkg/gtk/v4"
+
+	"krino/gui/internal/model"
+	"krino/internal/engine"
+)
+
+// historyView is the History & undo tab: the runs on the left, the selected
+// run's reversal on the right, and Undo (GUI design §4).
+type historyView struct {
+	w    *Window
+	root *gtk.Box
+
+	runs     *gtk.ListBox
+	reload   *gtk.Button
+	more     *gtk.Button
+	limit    int
+	runRows  []model.Run
+	selected int
+
+	header  *gtk.Label
+	note    *gtk.Label
+	list    *gtk.ListBox
+	selAll  *gtk.Button
+	selNone *gtk.Button
+	undo    *gtk.Button
+	cancel  *gtk.Button
+
+	tab      *model.UndoTab
+	cancelOp context.CancelFunc
+}
+
+// firstLimit is how many runs the list shows before Show more.
+const firstLimit = 50
+
+func newHistoryView(w *Window) *historyView {
+	h := &historyView{w: w, limit: firstLimit, selected: -1}
+	h.root = gtk.NewBox(gtk.OrientationVertical, 0)
+
+	h.reload = gtk.NewButtonWithLabel("Reload")
+	h.more = gtk.NewButtonWithLabel("Show more")
+	h.selAll = gtk.NewButtonWithLabel("Select all")
+	h.selNone = gtk.NewButtonWithLabel("None")
+	h.undo = gtk.NewButtonWithLabel("Undo")
+	h.undo.AddCSSClass("destructive-action")
+	h.cancel = gtk.NewButtonWithLabel("Cancel")
+
+	bar := gtk.NewBox(gtk.OrientationHorizontal, 6)
+	bar.SetMarginTop(6)
+	bar.SetMarginStart(6)
+	bar.SetMarginEnd(6)
+	bar.SetMarginBottom(6)
+	bar.Append(gtk.NewLabel("Runs"))
+	bar.Append(h.reload)
+	bar.Append(h.more)
+	h.note = gtk.NewLabel("")
+	h.note.SetXAlign(0)
+	h.note.SetHExpand(true)
+	h.note.SetEllipsize(pango.EllipsizeEnd)
+	h.note.SetMaxWidthChars(20)
+	bar.Append(h.note)
+	bar.Append(h.selAll)
+	bar.Append(h.selNone)
+	bar.Append(h.cancel)
+	bar.Append(h.undo)
+
+	h.runs = gtk.NewListBox()
+	h.runs.SetSelectionMode(gtk.SelectionSingle)
+	runScroll := gtk.NewScrolledWindow()
+	runScroll.SetChild(h.runs)
+	runScroll.SetSizeRequest(360, -1)
+
+	h.header = gtk.NewLabel("Select a run to see what undoing it would do.")
+	h.header.SetXAlign(0)
+	h.header.SetMarginStart(8)
+	h.header.SetMarginTop(6)
+	h.header.SetMarginBottom(6)
+	h.list = gtk.NewListBox()
+	h.list.SetSelectionMode(gtk.SelectionSingle)
+	listScroll := gtk.NewScrolledWindow()
+	listScroll.SetChild(h.list)
+	listScroll.SetHExpand(true)
+	listScroll.SetVExpand(true)
+	right := gtk.NewBox(gtk.OrientationVertical, 0)
+	right.Append(h.header)
+	right.Append(listScroll)
+
+	panes := gtk.NewPaned(gtk.OrientationHorizontal)
+	panes.SetStartChild(runScroll)
+	panes.SetEndChild(right)
+	panes.SetResizeStartChild(false)
+	panes.SetResizeEndChild(true)
+	panes.SetShrinkStartChild(false)
+	panes.SetPosition(360)
+	panes.SetVExpand(true)
+
+	h.root.Append(bar)
+	h.root.Append(gtk.NewSeparator(gtk.OrientationHorizontal))
+	h.root.Append(panes)
+
+	h.reload.ConnectClicked(func() { h.loadRuns() })
+	h.more.ConnectClicked(func() {
+		h.limit *= 4
+		h.loadRuns()
+	})
+	h.runs.ConnectRowSelected(func(row *gtk.ListBoxRow) {
+		if row != nil {
+			h.onRunSelected(row.Index())
+		}
+	})
+	h.selAll.ConnectClicked(func() { h.selectAll(true) })
+	h.selNone.ConnectClicked(func() { h.selectAll(false) })
+	h.undo.ConnectClicked(h.onUndo)
+	h.cancel.ConnectClicked(func() {
+		if h.cancelOp != nil {
+			h.cancelOp()
+		}
+	})
+	h.setBusy(false)
+	return h
+}
+
+// loadRuns reads the log and fills the run list.
+func (h *historyView) loadRuns() {
+	h.closeTab()
+	h.setBusy(true)
+	var runs []model.Run
+	h.cancelOp = runInBackground(func(ctx context.Context) error {
+		var err error
+		runs, err = model.Runs(h.w.engine, h.limit)
+		return err
+	}, func(err error) {
+		h.setBusy(false)
+		if err != nil {
+			h.w.setStatus("log: %v", err)
+			return
+		}
+		h.runRows = runs
+		h.selected = -1
+		h.fillRuns()
+		h.more.SetSensitive(len(runs) >= h.limit)
+		h.w.setStatus("%d run(s)", len(runs))
+	})
+}
+
+// fillRuns renders the run list, newest first.
+func (h *historyView) fillRuns() {
+	clearList(h.runs)
+	for _, r := range h.runRows {
+		h.runs.Append(runRowWidget(r))
+	}
+	h.clearPlan()
+}
+
+// runRowWidget is one line of the run list.
+func runRowWidget(r model.Run) *gtk.ListBoxRow {
+	box := gtk.NewBox(gtk.OrientationHorizontal, 8)
+	box.SetMarginStart(6)
+	box.SetMarginEnd(6)
+	box.SetMarginTop(2)
+	box.SetMarginBottom(2)
+	box.Append(column(r.Start.Format("2006-01-02 15:04"), 16, false))
+	box.Append(column(escape(runWhat(r)), 26, true))
+	row := gtk.NewListBoxRow()
+	row.SetChild(box)
+	return row
+}
+
+// runWhat is what a run did, for its line in the list.
+func runWhat(r model.Run) string {
+	var parts []string
+	if len(r.Dirs) > 0 {
+		parts = append(parts, strings.Join(r.Dirs, ", "))
+	}
+	if r.UndoOf != "" {
+		parts = append(parts, "undo of "+r.UndoOf)
+	}
+	parts = append(parts, r.Summary)
+	if r.Note != "" {
+		parts = append(parts, r.Note)
+	}
+	return strings.Join(parts, "  ")
+}
+
+// onRunSelected plans the reversal of the chosen run. Choosing an undo run
+// offers what is left of the run it reversed, as plain krino undo does.
+func (h *historyView) onRunSelected(i int) {
+	if i < 0 || i >= len(h.runRows) {
+		return
+	}
+	h.selected = i
+	r := h.runRows[i]
+	target := r.ID
+	h.note.SetText("")
+	if r.UndoOf != "" {
+		target = r.UndoOf
+		h.note.SetText("that run undid " + r.UndoOf + "; offering what is left of it")
+	}
+	h.closeTab()
+	h.setBusy(true)
+	h.header.SetText("planning the undo of " + target + "...")
+	var tab *model.UndoTab
+	h.cancelOp = runInBackground(func(ctx context.Context) error {
+		var err error
+		tab, err = model.PlanUndo(ctx, h.w.engine, target)
+		return err
+	}, func(err error) {
+		h.setBusy(false)
+		if err != nil {
+			h.header.SetText(escape(err.Error()))
+			h.w.setStatus("undo %s: %v", target, err)
+			return
+		}
+		h.tab = tab
+		h.fillPlan()
+		h.w.setStatus("undo %s: %d to reverse, %d refused",
+			tab.Run, tab.Counts.ToReverse, tab.Counts.Refused)
+	})
+}
+
+// fillPlan renders the undo plan: a row per file, refused ones marked and
+// never checkable.
+func (h *historyView) fillPlan() {
+	clearList(h.list)
+	if h.tab == nil {
+		return
+	}
+	// The header is the counts alone (GUI design §4); which run this is
+	// stands in the selected line on the left and in the status bar.
+	c := h.tab.Counts
+	if c.Files == 0 {
+		h.header.SetText("nothing left to reverse in " + h.tab.Run)
+	} else {
+		h.header.SetText(fmt.Sprintf("%d files · %d to reverse · %d refused",
+			c.Files, c.ToReverse, c.Refused))
+	}
+	for i, r := range h.tab.Rows {
+		h.list.Append(h.undoRowWidget(i, r))
+	}
+	h.updateUndoButton()
+}
+
+// clearPlan empties the right-hand side, between runs.
+func (h *historyView) clearPlan() {
+	clearList(h.list)
+	h.header.SetText("Select a run to see what undoing it would do.")
+	h.undo.SetLabel("Undo")
+	h.undo.SetSensitive(false)
+}
+
+// undoRowWidget is one file of the reversal.
+func (h *historyView) undoRowWidget(i int, r model.UndoRow) *gtk.ListBoxRow {
+	box := gtk.NewBox(gtk.OrientationHorizontal, 8)
+	box.SetMarginStart(6)
+	box.SetMarginEnd(6)
+	box.SetMarginTop(2)
+	box.SetMarginBottom(2)
+
+	check := gtk.NewCheckButton()
+	check.SetActive(r.Selected)
+	check.SetSensitive(r.Actable && !h.tab.Applied)
+	check.ConnectToggled(func() {
+		if h.tab != nil && h.tab.Rows[i].Selected != check.Active() {
+			h.tab.Toggle(i)
+			h.updateUndoButton()
+		}
+	})
+	box.Append(check)
+
+	box.Append(column(escape(r.Dir+"/"+r.File), 24, true))
+	// The outcome comes before the reversal: after an undo it is what the
+	// user is looking for, and the reversal is the column that ellipsizes.
+	if r.Outcome != "" {
+		box.Append(column(escape(r.Outcome), 12, false))
+	}
+	what := column(escape(undoWhat(r, h.w.dirRoot(r.Dir))), 30, false)
+	if r.Refused != "" {
+		what.AddCSSClass("error")
+	}
+	box.Append(what)
+	row := gtk.NewListBoxRow()
+	row.SetChild(box)
+	return row
+}
+
+// undoWhat is what would happen to one file, or why nothing will.
+// Destinations inside the directory are written relative to it.
+func undoWhat(r model.UndoRow, root string) string {
+	if r.Refused != "" {
+		return "refused: " + r.Refused
+	}
+	var parts []string
+	for _, s := range r.Steps {
+		if s.Refused != "" {
+			parts = append(parts, s.Action+" refused: "+s.Refused)
+			continue
+		}
+		if s.Dst == "" {
+			parts = append(parts, s.Action)
+			continue
+		}
+		parts = append(parts, s.Action+" "+shorten(s.Dst, root))
+	}
+	return strings.Join(parts, ", ")
+}
+
+// selectAll checks or unchecks every file that can be reversed.
+func (h *historyView) selectAll(on bool) {
+	if h.tab == nil {
+		return
+	}
+	if on {
+		h.tab.SelectAll()
+	} else {
+		h.tab.SelectNone()
+	}
+	h.fillPlan()
+}
+
+// onUndo reverses the checked files, off the main loop.
+func (h *historyView) onUndo() {
+	if h.tab == nil {
+		return
+	}
+	n := h.tab.SelectedCount()
+	h.setBusy(true)
+	h.w.setStatus("reversing %d file(s)...", n)
+	var res *engine.ApplyResult
+	h.cancelOp = runInBackground(func(ctx context.Context) error {
+		var err error
+		res, err = h.tab.Apply(ctx)
+		return err
+	}, func(err error) {
+		h.setBusy(false)
+		h.fillPlan()
+		h.undo.SetSensitive(false)
+		if err != nil {
+			h.w.setStatus("undo: %v", err)
+			return
+		}
+		if res == nil {
+			return
+		}
+		h.w.setStatus("%d reversed, %d failed, %d declined", res.Applied, res.Failed, res.Declined)
+		// The run list now says (undone); the plan stays as it is, showing
+		// each file's outcome.
+		h.refreshRunsKeepingPlan()
+	})
+}
+
+// refreshRunsKeepingPlan reloads the run list without dropping the plan the
+// user is looking at.
+func (h *historyView) refreshRunsKeepingPlan() {
+	runs, err := model.Runs(h.w.engine, h.limit)
+	if err != nil {
+		h.w.setStatus("log: %v", err)
+		return
+	}
+	h.runRows = runs
+	sel := h.selected
+	clearList(h.runs)
+	for _, r := range h.runRows {
+		h.runs.Append(runRowWidget(r))
+	}
+	h.selected = sel
+}
+
+// updateUndoButton keeps the button's label and state on the selection.
+func (h *historyView) updateUndoButton() {
+	if h.tab == nil {
+		h.undo.SetLabel("Undo")
+		h.undo.SetSensitive(false)
+		return
+	}
+	n := h.tab.SelectedCount()
+	h.undo.SetLabel(fmt.Sprintf("Undo %d selected", n))
+	h.undo.SetSensitive(!h.tab.Applied && n > 0)
+}
+
+// setBusy turns the buttons on or off around a background operation.
+func (h *historyView) setBusy(busy bool) {
+	h.reload.SetSensitive(!busy)
+	h.more.SetSensitive(!busy && len(h.runRows) >= h.limit)
+	h.runs.SetSensitive(!busy)
+	h.selAll.SetSensitive(!busy && h.tab != nil)
+	h.selNone.SetSensitive(!busy && h.tab != nil)
+	h.cancel.SetSensitive(busy)
+	if busy {
+		h.undo.SetSensitive(false)
+		return
+	}
+	h.updateUndoButton()
+}
+
+// closeTab drops the open undo plan and releases its locks.
+func (h *historyView) closeTab() {
+	if h.tab == nil {
+		return
+	}
+	if err := h.tab.Close(); err != nil {
+		h.w.setStatus("undo: %v", err)
+	}
+	h.tab = nil
+	h.clearPlan()
+}
+
+// clearList removes every row of a ListBox.
+func clearList(list *gtk.ListBox) {
+	for {
+		row := list.RowAtIndex(0)
+		if row == nil {
+			return
+		}
+		list.Remove(row)
+	}
+}
diff --git a/gui/internal/ui/plan.go b/gui/internal/ui/plan.go
index 8cbbdfd..fd2f563 100644
--- a/gui/internal/ui/plan.go
+++ b/gui/internal/ui/plan.go
@@ -263,7 +263,7 @@ func (p *planView) onScan() {
 	var tab *model.PlanTab
 	p.cancelOp = runInBackground(func(ctx context.Context) error {
 		var err error
-		tab, err = model.Plan(ctx, p.w.sess, d)
+		tab, err = model.Plan(ctx, p.w.engine, d)
 		return err
 	}, func(err error) {
 		p.setBusy(false)
@@ -342,13 +342,7 @@ func (p *planView) selectAll(on bool) {
 // fillList renders the rows: a checkbox, the file, what would happen, the
 // rule, and the outcome once applied.
 func (p *planView) fillList() {
-	for {
-		row := p.list.RowAtIndex(0)
-		if row == nil {
-			break
-		}
-		p.list.Remove(row)
-	}
+	clearList(p.list)
 	if p.tab == nil {
 		return
 	}
diff --git a/gui/internal/ui/window.go b/gui/internal/ui/window.go
index e35a363..c42a7e8 100644
--- a/gui/internal/ui/window.go
+++ b/gui/internal/ui/window.go
@@ -24,16 +24,17 @@ type Window struct {
 	win *gtk.ApplicationWindow
 
 	engine *engine.Engine
-	sess   *engine.Session
 
-	plan   *planView
-	status *gtk.Label
+	plan    *planView
+	history *historyView
+	status  *gtk.Label
 }
 
-// NewWindow builds the window for e; the session is the run everything in
-// it goes through.
-func NewWindow(app *gtk.Application, e *engine.Engine, sess *engine.Session) *Window {
-	w := &Window{app: app, engine: e, sess: sess}
+// NewWindow builds the window for e. Each plan and each undo is its own
+// run of krino, with its own run id in the log, so the window itself holds
+// no session.
+func NewWindow(app *gtk.Application, e *engine.Engine) *Window {
+	w := &Window{app: app, engine: e}
 	w.win = gtk.NewApplicationWindow(app)
 	w.win.SetTitle("krino")
 	w.win.SetDefaultSize(1000, 640)
@@ -41,8 +42,18 @@ func NewWindow(app *gtk.Application, e *engine.Engine, sess *engine.Session) *Wi
 	notebook := gtk.NewNotebook()
 	w.plan = newPlanView(w)
 	notebook.AppendPage(w.plan.root, gtk.NewLabel("Plan"))
-	notebook.AppendPage(placeholder("History and undo arrives with the next milestone."), gtk.NewLabel("History & undo"))
+	w.history = newHistoryView(w)
+	notebook.AppendPage(w.history.root, gtk.NewLabel("History & undo"))
 	notebook.AppendPage(placeholder("The rules editor arrives with a later milestone."), gtk.NewLabel("Rules"))
+	// The log is read when the tab is first opened, not at start-up: a
+	// window that only sorts never reads it.
+	loaded := false
+	notebook.ConnectSwitchPage(func(_ gtk.Widgetter, page uint) {
+		if page == 1 && !loaded {
+			loaded = true
+			w.history.loadRuns()
+		}
+	})
 
 	w.status = gtk.NewLabel("")
 	w.status.SetXAlign(0)
@@ -61,6 +72,7 @@ func NewWindow(app *gtk.Application, e *engine.Engine, sess *engine.Session) *Wi
 	// holds, rather than leaving a lock file for the next run to find.
 	w.win.ConnectCloseRequest(func() bool {
 		w.plan.closeTab()
+		w.history.closeTab()
 		return false
 	})
 	return w
@@ -69,6 +81,18 @@ func NewWindow(app *gtk.Application, e *engine.Engine, sess *engine.Session) *Wi
 // Show puts the window on screen.
 func (w *Window) Show() { w.win.Show() }
 
+// dirRoot is the path of the configured directory called name, or "" if
+// the configuration no longer has one - a log entry can outlive its
+// directory.
+func (w *Window) dirRoot(name string) string {
+	for _, d := range w.engine.Dirs {
+		if d.Name == name {
+			return d.Root
+		}
+	}
+	return ""
+}
+
 // setStatus writes the line at the bottom of the window.
 func (w *Window) setStatus(format string, args ...any) {
 	w.status.SetText(fmt.Sprintf(format, args...))