Microsoft is open sourcing the code of Windows Calculator on GitHub in a bid to encourage developers learn more about Microsoft technologies by reviewing the code used in the app. Developers can now use Windows Calculator functionality in their own apps and participate in code review and feedback via GitHub. Microsoft will continue to finally test and flight the app in Windows builds as usual.
https://www.notebookcheck.net/Microsoft-open-sources-the-Windows-Calculator-app-on-GitHub.413775.0.html