Hide leaving view after transition finishes

Editor assessment

The subject describes precise viewport behavior: the leaving view stays mounted until the transition finishes, which is a concrete, checkable claim in the router-outlet scope. fix(router-outlet) matches. No body, and for a timing fix of this size the subject plus pull reference suffices; nothing crucial is hidden. Wording is imperative and free of punctuation noise, though the subject is on the longer side. The message teaches the exact behavioral contract, which is more than many one-line fixes manage.