Ideally we could do this without session, but pragmatically
we still need it, because of the inflexible routing system,
and because of performance considerations.
Example: The tree is lazy loaded via a generic URL (admin/pages/treeview).
While we could add ?ID=<currentpage> to make the view (more or less) stateless,
it would trigger a full tree reload on every tree navigation action.
Instead, we assume that all "reachable" nodes are already cached,
and simply mark a different one as current. For this to work, we need
shared session state between CMS controllers.
See http://open.silverstripe.org/ticket/7815 for detail.
ENHANCEMENT Namespaced tree search parameters in CMSMain, in order to detect more reliably if a filter has been applied. Changing page search form to standard pushState behaviour, same as ModelAdmin (for both tree and list view).
MINOR Refactored list view loading
