fix(mediacontrols): always send an art URL, even when there is no art

Every other key in the MPRIS metadata map can be omitted safely,
because a client reading it renders a track with no title as a track
with no title. Art is different: KDE's applet treats an absent
mpris:artUrl as no news about the art and keeps drawing whatever the
last track had, so playing something without a cover left the previous
album's sleeve on screen — which reads as the wrong track playing
rather than as missing artwork.

The map's construction moves out of UpdateMetadata into a pure
metadataMap so it can be asserted on at all: everything else in this
file needs a live session bus, which is the same reason the Android
contract lives in an untagged file.

Closes #41

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/mediacontrols/mpris_linux.go

@@ -279,17 +279,19 @@ func (h *MPRISHandler) enqueue(fn func()) {
 	}
 }
 
-// UpdateMetadata pushes track metadata to D-Bus.
-func (h *MPRISHandler) UpdateMetadata(meta Metadata) {
-	h.mu.Lock()
-	h.trackID++
-	tid := h.trackID
-	h.mu.Unlock()
-
-	m := map[string]interface{}{
+// metadataMap builds the org.mpris.MediaPlayer2.Player Metadata value
+// for one track.
+//
+// It is separated from UpdateMetadata, which needs a live D-Bus
+// connection, so the map's contents can be asserted on: this file is
+// behind a build tag and everything in it that touches h is reachable
+// only from a session bus, which is the same reason the Android
+// contract lives in an untagged androidpayload.go.
+func metadataMap(meta Metadata, trackID uint64) map[string]any {
+	m := map[string]any{
 		"mpris:trackid": dbus.ObjectPath(
 			fmt.Sprintf(
-				"/org/yellowjacket/Track/%d", tid,
+				"/org/yellowjacket/Track/%d", trackID,
 			),
 		),
 	}
@@ -306,16 +308,45 @@ func (h *MPRISHandler) UpdateMetadata(meta Metadata) {
 		m["xesam:album"] = meta.Album
 	}
 
+	// Always present, even with nothing to point at.
+	//
+	// Every other key here can be omitted safely because a client
+	// reading the map sees a track with no title or no album and
+	// renders it that way. Art is different: KDE's applet (and
+	// others) treat an *absent* mpris:artUrl as "no news about the
+	// art" and keep drawing whatever the last track had, so playing
+	// something with no cover left the previous album's sleeve on
+	// screen — which reads as the wrong track playing rather than as
+	// missing artwork.
+	//
+	// An empty string is the honest answer and is what the spec's
+	// "URI" type degrades to; a client that cannot load it falls back
+	// to its own placeholder, which is the behaviour wanted.
+	artURL := ""
 	if meta.ArtFilePath != "" {
-		m["mpris:artUrl"] = "file://" + meta.ArtFilePath
+		artURL = "file://" + meta.ArtFilePath
 	}
 
+	m["mpris:artUrl"] = artURL
+
 	if meta.DurationSec > 0 {
 		m["mpris:length"] = int64(
 			meta.DurationSec,
 		) * usPerSec
 	}
 
+	return m
+}
+
+// UpdateMetadata pushes track metadata to D-Bus.
+func (h *MPRISHandler) UpdateMetadata(meta Metadata) {
+	h.mu.Lock()
+	h.trackID++
+	tid := h.trackID
+	h.mu.Unlock()
+
+	m := metadataMap(meta, tid)
+
 	h.enqueue(func() {
 		h.props.SetMust(playerIf, "Metadata", m)
 	})

backend/mediacontrols/mpris_linux_test.go

@@ -0,0 +1,86 @@
+//go:build linux && !android
+
+package mediacontrols
+
+import "testing"
+
+// The one key that must be present even when it is empty.
+//
+// Everything else in the map may be omitted, because a client reading
+// it renders a track with no title as a track with no title. Art is
+// different: KDE's applet treats an *absent* mpris:artUrl as no news
+// about the art and keeps drawing the last one it saw, so a track with
+// no cover wore the previous album's sleeve — which reads as the wrong
+// track playing rather than as missing artwork.
+func TestMetadataMapAlwaysCarriesArtURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		meta Metadata
+		want string
+	}{
+		{
+			name: "no art at all",
+			meta: Metadata{Title: "Blue in Green"},
+			want: "",
+		},
+		{
+			name: "art on disk",
+			meta: Metadata{
+				Title:       "Blue in Green",
+				ArtFilePath: "/covers/kind-of-blue_lg.jpg",
+			},
+			want: "file:///covers/kind-of-blue_lg.jpg",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			m := metadataMap(tt.meta, 1)
+
+			got, ok := m["mpris:artUrl"]
+			if !ok {
+				t.Fatal("mpris:artUrl is absent; it must always be sent")
+			}
+
+			if got != tt.want {
+				t.Errorf("mpris:artUrl = %v, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+// The trackid has to change between tracks or a client is entitled to
+// treat the metadata as describing the same track it already has.
+func TestMetadataMapTrackIDVaries(t *testing.T) {
+	t.Parallel()
+
+	first := metadataMap(Metadata{Title: "A"}, 1)["mpris:trackid"]
+	second := metadataMap(Metadata{Title: "B"}, 2)["mpris:trackid"]
+
+	if first == second {
+		t.Errorf("trackid did not change: %v", first)
+	}
+}
+
+// The optional keys stay optional — this is what makes artUrl's
+// always-present treatment a deliberate exception rather than drift.
+func TestMetadataMapOmitsEmptyOptionalFields(t *testing.T) {
+	t.Parallel()
+
+	m := metadataMap(Metadata{}, 1)
+
+	for _, key := range []string{
+		"xesam:title",
+		"xesam:artist",
+		"xesam:album",
+		"mpris:length",
+	} {
+		if _, ok := m[key]; ok {
+			t.Errorf("%s is present for an empty Metadata", key)
+		}
+	}
+}