But those things are overkill.
I think it could be as simple as replacing the winform icon with another that has the Overlay "Added / blended"; then lets windows do it's thing normally ;-), and then this will also have the "overlay" in the open app form as well ;-)
The position of the Overlay has to be programmable (Designer!), because some icons already have a lower Right indicator:
Once overlay images are done for the KryptonToolkit Items, #1205 Then also implement them for Form Icons in the task bar
https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/nf-shobjidl_core-itaskbarlist3-setoverlayicon
https://docs.devexpress.com/WPF/DevExpress.Mvvm.UI.TaskbarButtonService.OverlayIcon
https://docs.telerik.com/devtools/winforms/controls/taskbar-button/features/overlay-icon
https://docs.devexpress.com/WindowsForms/DevExpress.Utils.Taskbar.TaskbarAssistant.OverlayIcon
Let the App dev do it the same way as showing Taskbar progress: https://www.codeproject.com/Articles/37451/Display-Progress-and-Overlay-Icons-for-Multiple-Vi
But those things are overkill. I think it could be as simple as replacing the winform icon with another that has the Overlay "Added / blended"; then lets windows do it's thing normally ;-), and then this will also have the "overlay" in the open app form as well ;-)
The position of the Overlay has to be programmable (Designer!), because some icons already have a lower Right indicator: