Incorrect macOS (text) keyboard input behavior.

As shown in the screenshot, macOS works differently to other operating systems, holding down a key does not repeat the key (what is that useful for anyway, shouting?). On macOS it opens a menu with special symbols, similar to how it would work on iOS or Android.

Designing completely custom UI from scratch is difficult, you need to account for very tiny subtleties of each platform, and I suppose this was just missed.
The solution is not as simple as disabling the repetition on macOS (although that would be less problematic) because this is a setting that can be customised. I am not sure, but hopefully Cocoa has an API that can be used to query this setting, having duplicate settings inside an app that adjust system level functionality is incorrect according to the platform Human Interface Guidelines, and so not a solution.