Hello,
A feature on most modern browsers is the ability to "Stack" tabs. When a Tab titlebar is drag-and-hold onto another Tab, they are put into the same "horizontally cascading folder" within the Groupy group. Now that those tabs are stacked, the Tab titlebars collapse and merge together (only the first tab Title Is displayed when collapsed), but when clicked, the Stacked Tab titlebars expand out. This way, users can organize tabs into neat and compact stacks, and each stack automatically collapses when not being used.

^An example of how Vivaldi Browser handles Tab subgroups, The first tab (startpage) is not in a subgroup, the next 3 tabs are in an expanded subgroup, and the next 5 tabs are in a collapsed subgroup.
Saving a Groupy group would also save the position of all subgroups.
Hopefully this translates, as it is a bit hard to describe.
Thanks very much for your time and consideration and fantastic software !