Edit: here's a quick demo of ShareX for people who haven't used it [1]. You can see me enter capture mode (via keyboard shortcut), which starts in fullscreen capture mode, then I hover over the calculator to switch to window capture mode, then I move it away to switch back to fullscreen capture mode, then I click and drag to do a region capture and release to complete the capture. You can also see the annotation tools at top that can be used during capture.
[1]: https://giant.gfycat.com/WellwornSizzlingIrukandjijellyfish....