Does anyone have any ideas on how to take a long list of feature layers in your web map and group them? I want the groups to have check boxes like a regular layer.
You could perhaps make groups on map level, before to publish your portal.
Otherwise you could/should probably edit the LayerList widget.
I made a change not in order to group layers as you want, but to add section titles between layers.
I made this in ...\widgets\LayerList\LayerListView.js, under drawListNode function.
It's probably were you should have a look.
Best regards and good luck