Looks like it is available now, as at 0.41.1.
For composer, Ctrl + Shift + I and then drag the modal window to another screen.
For Ctrl + L chat, click on the Open Chat in Editor icon, and then drag the editor tab to another screen.
Edit: Hmm, Composer is working fine for me on another screen, but perhaps I spoke too soon about Chat on another screen, I seem to be getting undesired behaviour when I have the AI chat editor tab on another screen - when I ask for some suggestions, and then click ‘Apply’, it says ‘No file to apply to’ - still, a step in the right direction
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Also see this post for a demo of some of the new UI features: