In a surprise move, Microsoft has published the source code to the Windows Calculator on GitHub under the terms of the MIT License. The full source code, unit testing, and build system is available, as well as the roadmap heading forward.
Microsoft is allowing anyone to contribute to future development of Calculator. New features, bug fixes, and other changes can be submitted by all developers for potential inclusion into the release version. Even though Calculator is open source now, Microsoft will still be running its own testing for security and quality control. This is a large step forward for the open source community given that it may be the first time it is possible for developers to directly contribute to Windows 10 releases aside from finding bugs through the Windows Insider program.
Going forward, the 2019 roadmap does not include any groundbreaking advancements. Main focuses will be on improving the open source development process and keeping the app's fluent design up to date. New features are expected to include pinning the app on top of other windows as well as more customization options, likely referring to appearance.
Now that the calculator source code is available to everyone free of charge and eligible for commercial use, it may start popping up inside of other software. Possibilities are wide open for how creatively developers will put it to use.
Windows 10 is likely to remain closed source for the foreseeable future for practical reasons, but Microsoft's small embrace of open source software may expand to more applications over time. It may be wishful thinking, but Microsoft could certainly make more of its accessory applications open without introducing major risks to its platform.