This can be done by writing an app that uses Windows API hook functions.

It could be made to work with most apps - since hook functions intercept messages before the app gets them.

A simple "Mirror whatever I type into a given window's input" is not too hard to develop. But this is a serious security risk - what if someone set this up to some invisible window and grabbed everything you type?

Sure, many apps use hook functions (for example some remote desktop software), but it's a dicey thing to let some program hook everything that you type.

If you can trust the code, then it's cool