I don't know why you say goodbye window, I say hello window.
It's a book from the point of view of a little girl who goes to visit her grandparents, and there's a window where you can see the kitchen on your way to the front door. This means that sometimes she can see her grandparents in the kitchen before she actually gets to the door. For some reason, that's important to her.
She just details things that happen in her day. Some of the ideas are a little complex to be coming from such a small looking child. The images are very strange-looking. They are very stylized in an unusual way: scribble-like with weird colors that don't quite fall within the lines, like pastels. I'm not that big of a fan of the style.
I'm guessing it's popular because it's one of those books that, although not aggressively multicultural, depicts a multiracial family. Her parents are clearly different races, and so are her grandparents. That's nice to either help children in that situation have a character to identify with, as well as exposing children who aren't in that situation to different kinds of families.
The text is nothing special. There's no real story, just a series of events that happen when she visits her grandparents. It's kind of boring. I'm really not sure how something this banal came from the same author as The Phantom Tollbooth and The Dot and the Line, both fascinatingly weird stories. He's a good writer, but this isn't one of his best books. It's cute. It's not bad, just meh.