This method is called before a snip in the pasteboard is selected or deselected. If #f is returned, the selection change is disallowed. This method is not called when a selected snip is to be deleted (and thus de-selected indirectly); see also can-delete? .
See also on-select and after-select.
The editor is internally locked for writing when this method is called (see also Locks).
If on? is #t, then snip will be selected, otherwise snip will be deselected.