Apptorium has debuted SideNotes 1.0, a note-taking solution exclusively for macOS. 

It allows you to take notes and keep them out of the way by positioning notes on the side of your Mac’s screen. Notes can be shown or hidden with a single click, by simply pressing a keyboard shortcut or by moving mouse cursor. SideNotes provides text formatting that takes advantage of Markdown and regular text formatting.

It requires macOS 10.14 or later. During its launch, SideNotes 1.0 is available for $9.99 (50% off) exclusively through the Mac App Store in the Productivity category. A demo version is available ( Setapp subscribers also got access to SideNotes.