QGIS 3: Layout expression to get all features of a layer within the current map view

In a map layout template you can insert an expression which gets all feature's names of a certain layer, which are contained in the current layout's view/extent. For wxample you could concatenate all feature's names contained in the current map view like so:

aggregate(layer:='Corridors_86ef2ea9_d1a1_4d9f_9735_7b7b2fb54cb2', 
aggregate:='concatenate', 
expression:="Name", filter:=within($geometry, map_get( item_variables('Main Map'), 
'map_extent')), concatenator:=', ')