backend/mediacontrols/mpris_linux.go

@@ -286,12 +286,36 @@ func (h *MPRISHandler) UpdateMetadata(meta Metadata) {
 	tid := h.trackID
 	h.mu.Unlock()
 
+	m := metadataMap(meta, tid)
+
+	h.enqueue(func() {
+		h.props.SetMust(playerIf, "Metadata", m)
+	})
+}
+
+// metadataMap is the org.mpris.MediaPlayer2.Player Metadata dictionary
+// for one track.
+//
+// Absent fields are omitted, with one deliberate exception: a track
+// with **no cover art still publishes mpris:artUrl**, empty. Omitting
+// it is what a missing value naturally reads as, and it is wrong here
+// -- KDE's media controller keeps showing the last artwork it was
+// given, so playing an untagged track under a Plasma taskbar preview
+// displayed the cover of whichever earlier album happened to have one.
+// An empty string is our answer to "what is the artwork" rather than
+// no answer at all.
+//
+// A generated placeholder image would be the stronger fix and needs an
+// app icon to point at; this is the version that costs nothing and can
+// be checked with busctl.
+func metadataMap(meta Metadata, trackID uint64) map[string]interface{} {
 	m := map[string]interface{}{
 		"mpris:trackid": dbus.ObjectPath(
 			fmt.Sprintf(
-				"/org/yellowjacket/Track/%d", tid,
+				"/org/yellowjacket/Track/%d", trackID,
 			),
 		),
+		"mpris:artUrl": "",
 	}
 
 	if meta.Title != "" {
@@ -316,9 +340,7 @@ func (h *MPRISHandler) UpdateMetadata(meta Metadata) {
 		) * usPerSec
 	}
 
-	h.enqueue(func() {
+	return m
-		h.props.SetMust(playerIf, "Metadata", m)
-	})
 }
 
 // UpdatePlaybackState pushes the playback state and position

backend/mediacontrols/mpris_metadata_linux_test.go

@@ -0,0 +1,73 @@
+//go:build linux && !android
+
+package mediacontrols
+
+import "testing"
+
+// TestArtURLIsAlwaysAnswered pins the one field in the MPRIS metadata
+// dictionary that must be published even when it is empty.
+//
+// Omitting a field is what an absent value naturally reads as, and it
+// is wrong for artwork: KDE's media controller keeps showing the last
+// image it was handed, so a track with no cover art displayed the
+// cover of whichever earlier album had one — in the taskbar preview,
+// where the user has no way to tell it is stale.
+func TestArtURLIsAlwaysAnswered(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		meta Metadata
+		want string
+	}{
+		{
+			name: "art present",
+			meta: Metadata{
+				Title:       "Tideline",
+				ArtFilePath: "/covers/abc_lg.jpg",
+			},
+			want: "file:///covers/abc_lg.jpg",
+		},
+		{
+			name: "no art at all",
+			meta: Metadata{Title: "Untagged"},
+			want: "",
+		},
+	}
+
+	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 absent: a missing key " +
+					"leaves the previous track's art on screen")
+			}
+
+			if got != tt.want {
+				t.Errorf("mpris:artUrl = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+// TestEmptyFieldsStayAbsent guards the other half: artUrl is the
+// exception, not a new rule for the whole dictionary. A title of ""
+// published as "" would make every player show an empty line where it
+// would otherwise fall back to the filename.
+func TestEmptyFieldsStayAbsent(t *testing.T) {
+	t.Parallel()
+
+	m := metadataMap(Metadata{}, 7)
+
+	for _, key := range []string{
+		"xesam:title", "xesam:artist", "xesam:album", "mpris:length",
+	} {
+		if _, ok := m[key]; ok {
+			t.Errorf("%s should be omitted when it has no value", key)
+		}
+	}
+}