On iOS 11 and can confirm there’s a serious bug — I would guess it’s a multi-thread synchronization issue.
Seems like a pretty serious bug for a calculator to have! Everyone should definitely avoid the iOS 11 calculator until this is fixed.
It’s nothing so complicated. A UI element that’s animating (transitioning) can be configured to ignore further input until it’s done. The plus animation is long, and (probably oversight) ignores input until it’s done glowing. There’s no race, or lock up, or lag, or any of a dozen other explanations. The behavior is entirely deterministic, based solely on whether you tap + while glowing or not. (Still a stupid bug, but the fix is just changing a bit to allow more taps while animating.)
Somebody on the reddit thread said something that I found worth sharing: “I think calculators were figured out a while ago”.