I've attached a mockup of what this may look like via the dev tools in chrome, and was achieved as simply as adding the css attributes `position: sticky; z-index: 999;` to the `ul.redactor_toolbar` class on my local machine.
Whilst in the meantime, I could create an app that does this for me, it seems like overkill in the long-run.
Official response
Lara Proud
Hi Scott, thanks for this suggestion. We have implemented this change so the text formatting tools are now sticky in the reply box to ensure you don't have to scroll back to format text on longer messages.