Fixes#7546
SwiftUI uses type and structure to identify views, which can lead
to issues with tree like structures where the shape and type is the same
but the content changes. This was causing #7546.
To fix this, we need to add explicit identity to the split tree view
so that SwiftUI can differentiate when it needs to redraw the view.
We don't want to blindly add Hashable to SplitTree because we don't want
to take into account all the fields. Instead, we add an explicit
"structural identity" to the SplitTreeView that can be used by SwiftUI.
Sean Kelly