fix(evidence): X11 sensor polls so tab/title changes are tracked

The X11 source only re-read the active window on _NET_ACTIVE_WINDOW
changes, so switching a browser tab — which changes the window title
but not the active window — was invisible. All the time on the new tab
was credited to the stale title (e.g. reading a Consume article showed
up as time on the Keel tab).

Mirror the Windows sensor: poll the active window every 750ms and emit
only when the window or its title changes. A shared, display-free
pollLoop carries the change-dedup and is unit-tested for the exact
regression (a title-only change must emit).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/evidence/evidence.go

@@ -24,8 +24,9 @@ type WindowSnapshot struct {
 }
 
 // Source is the activity port. Watch runs until ctx is cancelled, invoking
-// onChange on every active-window change, and once immediately with the
+// onChange once immediately with the current window and then on every change of
-// current window.
+// the active window or its title (a tab switch changes the title but not the
+// active window).
 type Source interface {
 	Watch(ctx context.Context, onChange func(WindowSnapshot))
 }

internal/evidence/poll.go

@@ -0,0 +1,44 @@
+package evidence
+
+import (
+	"context"
+	"time"
+)
+
+// pollLoop samples the active window via read and forwards snapshots through
+// onChange, but only when the observation changes — a different window OR a
+// different title. The title case is the one that matters: switching a browser
+// tab changes the window title without changing the active window, so a sensor
+// that re-reads only on active-window events never sees it and credits the stale
+// title. Polling re-reads on a fixed cadence, so a tab switch is caught within
+// one interval.
+//
+// It emits once immediately, then samples every interval, until ctx is
+// cancelled. WindowSnapshot is comparable, so the change check is a plain
+// equality. Factoring the loop here keeps the change-dedup identical across
+// sensors and lets it be tested without a display. (The Windows sensor predates
+// this and keeps its own equivalent loop.)
+func pollLoop(ctx context.Context, interval time.Duration, read func() WindowSnapshot, onChange func(WindowSnapshot)) {
+	var last WindowSnapshot
+	var haveLast bool
+	emit := func() {
+		s := read()
+		if haveLast && s == last {
+			return
+		}
+		last, haveLast = s, true
+		onChange(s)
+	}
+
+	emit() // immediate current window
+	t := time.NewTicker(interval)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			emit()
+		}
+	}
+}

internal/evidence/poll_test.go

@@ -0,0 +1,95 @@
+package evidence
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestPollLoopEmitsOnTitleChangeWithinSameWindow is the regression for the bug
+// where the X11 sensor only re-read the window on _NET_ACTIVE_WINDOW changes and
+// so missed a browser tab switch — same window class, new title — crediting all
+// the time to the stale title. The poll loop must emit when only the title
+// changes.
+func TestPollLoopEmitsOnTitleChangeWithinSameWindow(t *testing.T) {
+	// Same window (class) throughout; the title changes mid-stream, like a tab
+	// switch. The repeated identical reads must NOT re-emit; the title change must.
+	titles := []string{"Keel - Brave", "Keel - Brave", "Consume - Brave", "Consume - Brave"}
+	var i int
+	read := func() WindowSnapshot {
+		idx := i
+		if idx >= len(titles) {
+			idx = len(titles) - 1
+		}
+		i++
+		return WindowSnapshot{Class: "Brave-browser", Title: titles[idx], Health: EvidenceHealth{Available: true}}
+	}
+
+	var mu sync.Mutex
+	var got []WindowSnapshot
+	onChange := func(s WindowSnapshot) {
+		mu.Lock()
+		got = append(got, s)
+		mu.Unlock()
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() { pollLoop(ctx, time.Millisecond, read, onChange); close(done) }()
+
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		mu.Lock()
+		n := len(got)
+		mu.Unlock()
+		if n >= 2 {
+			break
+		}
+		if time.Now().After(deadline) {
+			cancel()
+			<-done
+			t.Fatalf("expected >=2 emits (one per distinct title), got %d", n)
+		}
+		time.Sleep(time.Millisecond)
+	}
+	cancel()
+	<-done
+
+	mu.Lock()
+	defer mu.Unlock()
+	if got[0].Title != "Keel - Brave" {
+		t.Fatalf("first emit title = %q, want %q", got[0].Title, "Keel - Brave")
+	}
+	if got[1].Title != "Consume - Brave" {
+		t.Fatalf("second emit title = %q, want %q (a title-only change must emit)", got[1].Title, "Consume - Brave")
+	}
+}
+
+// TestPollLoopDedupesIdenticalObservations confirms an unchanged observation is
+// not re-emitted, so redundant samples never churn the consumer.
+func TestPollLoopDedupesIdenticalObservations(t *testing.T) {
+	read := func() WindowSnapshot {
+		return WindowSnapshot{Class: "code", Title: "main.go", Health: EvidenceHealth{Available: true}}
+	}
+	var mu sync.Mutex
+	var n int
+	onChange := func(WindowSnapshot) {
+		mu.Lock()
+		n++
+		mu.Unlock()
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() { pollLoop(ctx, time.Millisecond, read, onChange); close(done) }()
+	time.Sleep(50 * time.Millisecond)
+	cancel()
+	<-done
+
+	mu.Lock()
+	defer mu.Unlock()
+	if n != 1 {
+		t.Fatalf("identical observations must emit once, got %d emits", n)
+	}
+}

internal/evidence/x11.go

@@ -4,24 +4,29 @@ package evidence
 
 import (
 	"context"
+	"time"
 
-	"github.com/jezek/xgb/xproto"
 	"github.com/jezek/xgbutil"
 	"github.com/jezek/xgbutil/ewmh"
 	"github.com/jezek/xgbutil/icccm"
-	"github.com/jezek/xgbutil/xevent"
-	"github.com/jezek/xgbutil/xprop"
-	"github.com/jezek/xgbutil/xwindow"
 )
 
+// pollInterval is how often the X11 sensor samples the active window. ~1s
+// latency on a switch is immaterial for a focus tracker, and polling is what
+// catches a title change inside the same window — e.g. switching a browser tab,
+// which changes the title but not the active window, so an _NET_ACTIVE_WINDOW
+// event subscription would miss it and credit the stale title. Mirrors the
+// Windows sensor's polling cadence.
+const pollInterval = 750 * time.Millisecond
+
 // NewSource returns the real X11 active-window sensor.
 func NewSource() Source { return &x11Source{} }
 
 type x11Source struct{}
 
-// Watch opens one long-lived X connection, subscribes to _NET_ACTIVE_WINDOW
+// Watch opens one long-lived X connection and samples the active window every
-// changes on the root window, and emits a snapshot immediately plus on every
+// pollInterval, emitting a snapshot whenever the active window OR its title
-// change. Any failure degrades to an Unavailable snapshot; it never panics the
+// changes. Any failure degrades to an Unavailable snapshot; it never panics the
 // daemon.
 func (s *x11Source) Watch(ctx context.Context, onChange func(WindowSnapshot)) {
 	X, err := xgbutil.NewConn()
@@ -32,38 +37,11 @@ func (s *x11Source) Watch(ctx context.Context, onChange func(WindowSnapshot)) {
 	}
 	defer X.Conn().Close()
 
-	root := X.RootWin()
+	pollLoop(ctx, pollInterval, func() WindowSnapshot { return snapshot(X) }, onChange)
-	activeAtom, err := xprop.Atm(X, "_NET_ACTIVE_WINDOW")
-	if err != nil {
-		onChange(unavailable("no _NET_ACTIVE_WINDOW atom: " + err.Error()))
-		<-ctx.Done()
-		return
-	}
-
-	// Listen for property changes on the root window.
-	if err := xwindow.New(X, root).Listen(xproto.EventMaskPropertyChange); err != nil {
-		onChange(unavailable("cannot listen on root window: " + err.Error()))
-		<-ctx.Done()
-		return
-	}
-
-	emit := func() { onChange(snapshot(X)) }
-	emit() // immediate current window
-
-	xevent.PropertyNotifyFun(func(_ *xgbutil.XUtil, ev xevent.PropertyNotifyEvent) {
-		if ev.Atom == activeAtom {
-			emit()
-		}
-	}).Connect(X, root)
-
-	// Run the event loop until ctx is cancelled.
-	go func() {
-		<-ctx.Done()
-		xevent.Quit(X)
-	}()
-	xevent.Main(X)
 }
 
+// snapshot reads the current active window's title and class. A missing active
+// window (or read error) yields an Unavailable snapshot.
 func snapshot(X *xgbutil.XUtil) WindowSnapshot {
 	active, err := ewmh.ActiveWindowGet(X)
 	if err != nil || active == 0 {