By writing a whole new one without considering the 20%. It's also missing a lot of features compared to the old one. (Features that may not matter to everyone, granted, but I found that there were more things that weren't there for my typical uses than ones that were.) Installing the old calculator[1] isn't much of a problem, though - although you do need to pin it to make it easily accessible without finding yourself opening the new uselessness accidentally about 50% of the time.
They wrote a new calculator for Win10, but AFAIK this breakage wasn't because they actually rewrote it again (as evidenced by the fact that it hasn't changed in any other obvious ways), but because of some subtle and as-yet-unknown change --- exactly the type of thing that horrifies me about silent automatic updates. What used to work is now subtly broken, and no one knows why.
Googling around, I found another discussion on it:
If you're going to add something, add something better than the old Calc, unless you really need some feature that only exists on that Calc. I use their PowerCalc Powertoy for Windows XP. [1] There's also SpeedCrunch which in some ways is probably better (minus the plotting) but personally I'm used to PowerCalc.
(No, not the new abomination of the same name that comes up when you search for "Calculator Plus", but the original XP-era one that is now only available from archive.org. Irritatingly enough, MS removed the old --- and better --- one when they introduced the new one.)
It's easier to get than finding an XP installation or ISO to extract a calc.exe from. The installer is also signed by MS, which may be a factor in some highly locked-down corporate environments.
It wouldn't surprise me - I'll often do the same with Python on non-Windows systems rather than bothering with the native calculator; I'm not even sure if Linux has one (or if it's the same across distros), and Python is ubiquitous, fast, and has familiar syntax.
Personally I used to use Maple for this kind of thing circa 2002. It used to launch on Windows in a fraction of a second (faster than the built in calculator tool!), and was an amazing calculator for pretty much any purpose.
Unfortunately some version of Maple around that time switched to a Java-based front end, and thereafter took forever to load, and had a UI littered with glitches.
I've used J (a language in the APL lineage) as an interactive calculator before. It makes some things pretty easy, especially if you need to operate on series of numbers.
What's the sum of the numbers from 15 to 20 inclusive?
i.6 N.B. Returns an array 0 1 2 3 4 5
15 + i.6 N.B. Returns an array 15 16 17 18 19 20
+/ 15 + i.6 N.B. returns the number 105
edit: Or course, if we're talking about languages helpful for calculation, we simply must include a reference to https://frinklang.org/
They're learning from GNOME! (GNOME whatever the latest version includes a new calculator that is an awful step backwards compared to the old one, luckily I can still install the old one).
And it is slower, really noticeably slower in terms of startup time than the old calc.exe in windows 7 and before. Not to mention it covers 50% of the screen for no reason.
If the new one just wraps the old one, but the new one breaks on certain inputs, how is it breaking? Maybe the outer "new" layer uses a new string to number parser than the inner "old" layer used to use, and that's where the bugs are being introduced?
They continued to update the underlying engine with new features and bug fixes and new bugs, they just didn't rewrite it from scratch.
Apparently a recent update also fixed a longstanding problem where the square root of a perfect square integer doesn't give you the exact right answer, so maybe there was a general change to the implementation of exponents and roots which may have introduced this bug too.
[1] https://winaero.com/download.php?view.1795