M1: X11 active-window adapter + platform fallback

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

internal/evidence/x11.go

@@ -0,0 +1,94 @@
+//go:build linux
+
+package evidence
+
+import (
+	"context"
+	"log"
+
+	"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"
+)
+
+// 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
+// changes on the root window, and emits a snapshot immediately plus on every
+// change. 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()
+	if err != nil {
+		onChange(unavailable("cannot connect to X server: " + err.Error()))
+		<-ctx.Done()
+		return
+	}
+	defer X.Conn().Close()
+
+	root := X.RootWin()
+	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)
+}
+
+func snapshot(X *xgbutil.XUtil) WindowSnapshot {
+	active, err := ewmh.ActiveWindowGet(X)
+	if err != nil || active == 0 {
+		return unavailable("no active window")
+	}
+	title, err := ewmh.WmNameGet(X, active)
+	if err != nil || title == "" {
+		// Fall back to ICCCM WM_NAME.
+		if t, e := icccm.WmNameGet(X, active); e == nil {
+			title = t
+		}
+	}
+	var class string
+	if wmClass, err := icccm.WmClassGet(X, active); err == nil && wmClass != nil {
+		class = wmClass.Class
+	}
+	return WindowSnapshot{
+		Title:  title,
+		Class:  class,
+		Health: EvidenceHealth{Available: true},
+	}
+}
+
+func unavailable(reason string) WindowSnapshot {
+	log.Printf("evidence: %s", reason)
+	return WindowSnapshot{Health: EvidenceHealth{Available: false, Reason: reason}}
+}