Under what circumstances can a judge issue a judgement for possession when a tenant vacated the premises prior to trial, returned the keys to landlord and made it clear that he no longer lives at the premises?
Does a judge hold a legal right to issue a judgement for possession in such circumstances?
There are multiple potential issues involved with an eviction. The primary interest here seems to be landlord repossession of the premise; there is also potentially collection of rents owed, and official termination of the tenancy. This identifies three landlord interests, the latter two of which are not addressed by tenant leaving. Tenant may have unlawfully broken the lease. Landlords have limited rights over their property, and cannot "evict or fail to renew a lease, whether it is a written or an oral lease without good cause" (a matter that the court decides). The landlord's suit can result in "A Judgment for Possession [which] ends the tenancy and allows the landlord to have the tenant evicted from the rental premises" – the suit thus accomplishes two things. If a tenant (later) pays the owed rent, the tenancy still exists and tenant can move back in. This is the interpretation that is most favorable to the landlord.
80 West Century LLC v. Drossos Lorenzo recites in §II numerous prior rulings regarding mootness, that "Mootness is a threshold justiciability determination rooted in the notion that judicial power is to be exercised only when a party is immediately threatened with harm", that "Courts 'normally will not entertain cases when a controversy no longer exists and the disputed issues have become moot'", and that "An issue is 'moot when [the] decision sought in a matter, when rendered, can have no practical effect on the existing controversy'".
Furthermore, "In a summary dispossess action, 'the court's jurisdiction is limited to determining the issue of the landlord's right to possession of the premises'", citing Daoud v. Mohammad. Specifically,
The landlord's repossession of the premises, either by execution of a warrant for removal or a voluntary vacation of the premises, renders moot an appeal from the JOP. Accordingly, we ordinarily dismiss as moot an appeal challenging an eviction when the tenant was removed from or otherwise vacated the premises.
In Daoud, the court states that
Because the court's jurisdiction is limited to determining the issue of the landlord's right to possession of the premises, and, as previously noted, the tenant vacated the premises and the premises have been re-rented, the issue can no longer be determined.
In 80 West Century, citing Daoud, the court notes that
An evicted tenant may seek in the Law Division damages arising from a wrongful eviction
which identifies what controls mootness: court jurisdiction. Special Civil Court is a more-efficient limited jurisdiction court. As stated in Daoud,
such actions [in the Special Civil Part] are statutory proceedings designed to accord landlords an expeditious and inexpensive means of regaining possession of leased premises as authorized by statute.
If the only issue is repossession and plaintiff selects Special Civil Court, then the motion is moot.
I write this very tentatively, given my unfamiliarity with the application of this doctrine in practice and invite edits to improve this answer.
The voluntary-cessation doctrine is an exception to mootness. A defendant cannot render a case moot simply by ceasing their illegal conduct. See United States v. W.T. Grant Co., 345 U.S. 629 (1953) (citations removed):
... voluntary cessation of allegedly illegal conduct does not deprive the tribunal of power to hear and determine the case, i. e., does not make the case moot. A controversy may remain to be settled in such circumstances, e.g., a dispute over the legality of the challenged practices. The defendant is free to return to his old ways. This, together with a public interest in having the legality of the practices settled, militates against a mootness conclusion. For to say that the case has become moot means that the defendant is entitled to a dismissal as a matter of right. The courts have rightly refused to grant defendants such a powerful weapon against public law enforcement.
...
Along with its power to hear the case, the court's power to grant injunctive relief survives discontinuance of the illegal conduct.
