Thank you for your answer.
"- Even on a large (27") screen, there were times when the bottom of a long inspector disappeared under the library pane. I assume this happened more often on a MacBook-size screen. The space wasn't really empty, a lot of the time."
You're right. I guess I was biased by my experience with large monitors.
"— The library palette (for controls and views) contains a lot of items. It's usually easiest to type in the filter field to find the thing you're looking for. The new window can be called up with a command-key equivalent (Command-Shift-L), and then you can just keep typing to enter your filter. If you can get used to that workflow, you won't really notice that it's floating."
I don't care too much if it's floating but I'd prefer to have an option to pin it to a panel or avoid it from disappear (I know I can press the alt key while drag some controls).
"— The new library window is context sensitive. You no longer see code snippets while editing storyboards, and you no longer see UI controls when editing source code.
— To create a code snippet, use Editor->Create Code Snippet (or right-click on the selected text). After that, it's the same snippet editor as before."
Yes, I know that, but still, I often drag SceneKit nodes to the library in order to reuse later and I can't see the alternative with this new behavior (and I guess there probably will be one). That was the reason for asking. I suppose we'll have to wait to next releases to see how it evolves.