Hello,
Being a C# developer, recently, I worked on a similar application that required displaying ticker notifications (just like the information banner mentioned in the project description), along with an alert and a popup message, obtained from the central server using the given REST API. The application was developed using C# and WPF. I also developed the installer package for the application using The WiX Toolset.
As it seems that you want to develop a cross-platform application to display the information banner at all times on the screen, I researched a bit and found that we can use the appbar (Application Desktop Toolbar) for this purpose. Alternatively, so as to simplify the development process, we can PInvoke the base API in C#. Moreover, I also found a simple UI application in WPF that implements the required functionality. The given application works as expected in all conditions (i.e. when the user maximizes the existing window, opens YouTube video in full-screen, etc.) However, it seems that this application is failing when playing a video, say using Windows Media Player in full-screen. So, if needed, then we may have to spend some more time so as to make it work in full-screen mode also.
Further, I would like to suggest using the UNO WinUI framework (from Microsoft) to target all the given platforms (i.e. Windows, MacOS, and Linux) from a single codebase.
Feel free to reply to this proposal for any further queries/suggestions.
Regards.