The original poster showed a photo where part of the broken edge is shiny. That shows that the cracking started there, and the edges rubbed together before the rest let go.
I'd take a close look at the steel in the liftgate for any cracks. If it starts cracking but not yet torn away, get it fixed by a good body shop. The heat from welding will be too hot for glass that close, so removing and reinstalling the glass costs are necessary. And, the paint gets burned, so repainting is another expense.
I'd take a close look at the steel in the liftgate for any cracks. If it starts cracking but not yet torn away, get it fixed by a good body shop. The heat from welding will be too hot for glass that close, so removing and reinstalling the glass costs are necessary. And, the paint gets burned, so repainting is another expense.