Inconsistent/wrong musical keys in the UI

[fwcd]

Bug Description

Switching the key notation mode (e.g. Traditional <-> Lancelot/Traditional) causes the key to change, additionally jumping down/up a semitone doesn't always land on the starting key, especially after changing the notation mode. See the video for a short demo:

demo.mp4

Anyone's got an idea what the issue could be?

Version

2.5-alpha-114-g75ac186cfa

OS

macOS 14.0

[fwcd]

I suspect this to be some kind of rounding error, but I'll have to investigate it more deeply

[fwcd]

Dug around a little more and found that my confusion around the implementation of WKey actually was the source of the issue. Notice how each key notation always produces the same key in the UI:

Screen.Recording.2023-10-01.at.19.14.35.mov

The issue is that the key of the track is set to the key notation index, which it obviously shouldn't be. Here's the problematic line (which seems to have been introduced as early as 2019):

mixxx/src/widget/wkey.cpp

Lines 65 to 67 in 0119d85

	void WKey::keyNotationChanged(double dValue) {
	setValue(dValue);
	}

Should be easy-ish to fix though. Perhaps we should just use more descriptive variable names (and perhaps avoid passing enum indices as doubles?).

cc @daschuer

[fwcd]

Fixed as of #12051