Interesting request! I'm torn on whether this is a TreeView request or a DataGrid request so adding both @predavid and @harinikmsft as the respective feature owners. There's also an ItemsView prototype being worked on in the feature/ItemsView branch and maybe this could be an interesting addition there as well. @micahl @ranjeshj for awareness on that.

@micahl - given the investments in the new ItemsView , this request would be a really nice addition there.

Is this it's own control, or an item type for the DataGrid control?

For reference, in the past, we used this type of control in WPF in Microsoft Message Analyzer. Think it was built custom from the Silverlight DataGrid control? I don't remember, it was a long time ago now.

I would vote towards this being a Column Type or Grid Type added to the DataGrid control.

This is something I'm exploring with my DataTable experiment in the Windows Community Toolkit Labs, see that discussion here: CommunityToolkit/Labs-Windows#415 (comment)

(Still just an early prototype, so indentation isn't corrected for other columns like in OP's image.)

Though realized that Header/Footer weren't exposed for TreeView either for how it's currently working in our prototype and filed #8451 as well.