When a sheet appears, it swaps place with the keyboard. The toolbar never changes position; the sheets are always the same size as the keyboard. Compare this to the X/Twitter composer, where the Add Media gallery is full-screened. Use modals sparingly!
Speaking of things you’ve seen before, take a look at what happens when you remove items from the attachment grid. It’s fairly similar to what you’d see on X, but they animate between layouts here, which is always a nice touch.