I'd find it helpful if the Pianos in the Mixer Window would have a small dot on the Middle C key.

The same counts for the Big Piano Window, but I can imagine that for some purposes a user might explicitly not want it there - so for the Big Piano I suggest it just as an option of the Big Piano Window Preferences.

That's a reasonable suggestion/request.

Middle C (the fourth C key from left on a standard 88-key piano keyboard) has significant relevance in music. Specifically identifying it might help others, especially when a smaller keyboard layout might not make this so obvious.

exactly, including a shifted keyboard frame and such

A problem is that there are several competing definitions of Middle C. This feature would have to be user-definable.

Hey Matt!

I'm not sure what you mean, since there is only one key on the piano keyboard that is - I'd think quite commonly - defined as middle key. Or do you mean in regard to written/sounding notes for horn players?

Well, you are correct about horn players. But some keyboard vendors define Middle C differently. Here is a good summary: https://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=338513

Ah ok - then, sure, it should be user-definable in order to also match the needs of horn players etc. :-)

I was looking for away to mark it the image but it all uses the same image.