So, thanks for taking the time to answer. I am, however, still stuck. Somehow it only draws the line on top of the track titles, rather than actually affecting the size of the group.
I figure any chance of getting this resolved rests on sharing my (simplified) config:
"Track List":
//Text Colors
$puts(text_color,$if(%el_isplaying%,%playing_text_color%,%text_color%))
$puts(text_color_alt,$if(%el_isplaying%,%playing_text_color%,%text_color_alt%))
$puts(text_color,$if(%el_selected%,%selected_text_color%,$get(text_color)))
$puts(text_color_alt,$if(%el_selected%,%selected_text_color%,$get(text_color_alt)))
//Track Number
$font(,10,)
$drawtextex($if2(%tracknumber%,00),$add(%art_size%,15),0,30,%el_height%,$get(text_color_alt),hcenter vcenter)
//Track Title
$drawtextex([%track artist%' - '][%title%],$add(%art_size%,50),0,$sub(%el_width%, $add(%art_size%,195)),%el_height%,$get(text_color),vcenter end_ellipsis)
//Track Length
$drawtextex(%length%,$sub(%el_width%,35),0,90,%el_height%,$get(text_color_alt),left vcenter end_ellipsis)
//Cover
$enabledraw(1)
$setworldtransform(2)
$imageabs(5,-47,%art_size%,%art_size%,$directory_path(%path%)\Cover.png,artreader)
"Group Header":
//Title Color
$puts(title_color,$if(%el_selected%, %selected_text_color%, %text_color%))
$puts(title_color2,$if(%el_selected%, %selected_text_color%, %text_color2%))
//Group Title
$font(,18,)
$drawtextex($if2(%album%,Unknown),322,-5,880,%el_height%,$get(title_color),end_ellipsis vcenter)
$font(,14,)
$drawtextex($if2(%date%,),322,15,880,%el_height%,$get(title_color2),end_ellipsis vcenter)
//Cover
$enabledraw(1)
$setworldtransform(1)
$imageabs(5,5,%art_size%,%art_size%,$directory_path(%path%)\Cover.png,artreader)
Row height: 26
Row number of group header: 2
Minimum row number of group: $ifgreater(%el_direct_item_count%,1,$add(%el_item_count%,1),2)
Really hope to get this resolved, and thanks again for bearing with me.