What is the best way to create a smooth notification window in C #?

Question

... similar to those created by email clients such as thunderbird or Outlook, sliding or disappearing from the tray.

Answer 1

What you are looking for is a Windows shell notification. There are quite a few implementations available.

Here is the MSDN manual and QuickStart link.

https://docs.microsoft.com/en-us/windows/desktop/shell/notification-area
https://docs.microsoft.com/en-us/windows/desktop/shell/quickstart-sending-desktop-toast

And here is an example implementation.
https://code.msdn.microsoft.com/windowsdesktop/sending-toast-notifications-71e230a2