main.kv

<SongScreen>:
    name: 'song_screen'
    BoxLayout:
        orientation: 'vertical'
        id: song_screen_bg

        NowPlayingAlbumArt:
            size: self.texture_size
            size_hint: None, None
            elevation: 10
            pos_hint: {"center_x": .5, "center_y": .5}
            id: album_art

        BoxLayout:
            orientation: 'horizontal'
            size_hint_y: None

            MDLabel:
                id: song_cur_pos_label
                size_hint: 0.2, 1
                halign: "center"
            MDSlider:
                id: song_slider
                min: 0
                max: root.song_max_length
                value: root.song_current_pos
                hint: False
                on_touch_up: root.seek_song()  # HERE
            MDLabel:
                id: song_total_length_label
                size_hint: 0.2, 1
                halign: "center"

        MDBottomAppBar:
            MDToolbar:
                id: song_screen_toolbar
                title: root.song_name
                icon: root.song_state
                type: 'bottom'
                left_action_items: [["android", lambda x: root.go_to_main()]]
                on_action_button: root.toggle_song_play()
                mode: 'center'