This isn’t currently possible, sorry. Notebook content is not allowed to overlay system content; this makes it harder for user-generated notebook content to masquerade as part of the system user interface. It might be possible to support this in some limited way in the future (say by having a special sticky cell), but we don’t have any current plans to support it.
You can using position-sticky and position-fixed elements when you embed your notebook on your own site, where you control how your notebook is displayed.