She clearly felt that it would be tactless for them to remain the room while Harry and the Dursleys exchanged loving, possibly tearful farewells.
Ron got back off the bed, put his arm around her once more, and frowned at Harry as though reproaching him for lack of tact.
Harry could not think of anything to say, not least because it was highly unusual for Ron to be teaching anyone else tact.
"Albus Severus," Harry said quietly, so that nobody but Ginny could hear, and she was tactful enough to pretend to be waving to Rose, who was now on the train, "you were named for two head-masters of Hogwarts.
There are no more uses of "tact" in the book.
Show samples from other sources
He was a big, good man with a soft baritone, who ruled with easy tact a choir of repressed soloists, and who had an unerring memory for the favorite hymns of District Superintendents.