Why is it vanishing instead of vanished?

Question

The Vanishing Glass

. . . . . .

Dudley came waddling toward them as fast as he could. “Out of the way, you,” he said, punching Harry in the ribs. Caught by surprise, Harry fell hard on the concrete floor. What came next happened so fast no one saw how it happened — one second, Piers and Dudley were leaning right up close to the glass, the next, they had leapt back with howls of horror. Harry sat up and gasped; the glass front of the boa constrictor’s tank had vanished. The great snake was uncoiling itself rapidly, slithering out onto the floor. People throughout the reptile house screamed and started running for the exits. As the snake slid swiftly past him, Harry could have sworn a low, hissing voice said, “Brazil, here I come… Thanksss, amigo.” (Harry Potter and the Sorcerer's Stone)

From the following content, it seems like The Vanished Glass is more suitable than the title. What does the title mean?

Answer 1

This passage is about a surprising event: one moment the glass is present, the next moment the glass is absent. This sudden event is the glass vanishing. Thus the important feature of the glass is that it is a vanishing glass: it is a glass that vanishes.

A vanished glass would be a glass that was present in the past, and is no longer there now. This description applies to a vanishing glass, but it is less precise, and a vanished glass is not in itself a surprising fact, unlike a vanishing glass. A vanished glass could have been taken away by someone while nobody was looking. A vanishing glass, on the other hand, is a decidedly supernatural event.

Answer 2

During the time of the chapter, the glass vanishes, so the title uses a present participle. A title of "the vanished glass" makes it sound as though the glass disappeared before the action of the chapter started.