Microsoft rolled out Windows 10 build 18956 to the Fast ring late last week, introducing an Always on Top mode for the Calculator app, among other new updates. That particular feature was not immediately available at the time, but today it appears to have gone live for Windows 10 Insiders.
As detailed by Microsoft when announcing this feature, the new Calculator app mode allows you to set it on top of everything else on your PC display. Switching to that mode is pretty straightforward: you just have to click on the icon sitting next to the Standard mode name, which is the default view.
Once in Always on Top mode, the app remains on display no matter which window you navigate to. It's an ideal capability for when you're doing some calculations for your weekend travel expenses, for example.
Of course, you can still return to the Standard mode for a larger view of Calculator. It's the newest improvement received by the app since it was open-sourced by Microsoft in March of this year, made available via GitHub for developers to improve. Shortly after that, the app received a graphing mode, a feature that's especially useful to algebra students.
Source: Leopeva64-2 (Reddit)