After filtering my document to a sparse tree, how do I undo the filter, going back to the previous view/visibility? Because I haven't worked out how to do this out I have to make everything visible, which isn't nice in big documents. Seems like it must be obvious but I can't find the answer...
This looks like it might be possible to me. In the following code I use some advice to save the outline state before generating the sparse tree, and to restore it when it is cleared via C-c C-c.
(setq lexical-binding t)
(let ((*outline-data* nil))
(defun org-save-outline-state (&optional arg type)
(setq *outline-data* (org-outline-overlay-data t)))
(defun org-restore-outline-state (&optional arg)
(when *outline-data*
(org-set-outline-overlay-data *outline-data*)
(setq *outline-data* nil))))
(advice-add 'org-sparse-tree :before 'org-save-outline-state)
(advice-add 'org-match-sparse-tree :before 'org-save-outline-state)
(advice-add 'org-ctrl-c-ctrl-c :after 'org-restore-outline-state)
It seems to do what you want.