Comment 13 for bug 1760316

> Let's try to go on the same page ..

Sure :) Back to square one, I just tested this with a default mapping:

Effects can (by default) only be focused if the respective unit is expanded. Logic is reversed in the actual behaviour: when the focus button on a controller is pressed the unit is automatically expanded.
IMO @be was right to implement this safety switch in https://github.com/mixxxdj/mixxx/pull/1211 so effects can not be focused by accident. There'd be immidiate on-screen feedback.
'focused_effect' is set to 0 (controller knobs = GUI meta knobs) when collapsing and re-set to [previously focused_effect] again when the units is expanded later-on.

Later in the same PR, @be implemented 'allowFocusWhenParametersHidden' (default FALSE) on my request so that users can explecitely allow units to show focused effects WITHOUT showing the parameters.
This is helpful to control 'simple' effects like filters without wasting screen space.

So IMO this is not a mappping issue.

As you can see, neither 'focused_effect' nor 'show_focus' cannot be used to highlight the focused unit in a default 4-fx-unit setup because only users who explicitely enabled 'allowFocusWhenParametersHidden' would see the highlight with collapsed units.

What I wish is a highlight CO that can be connected to both expanded AND collapsed units so that it's useful for ANY mapping, both default ('allowFocusWhenParametersHidden' FALSE) or customized ('allowFocusWhenParametersHidden' TRUE).
As far as I can tell, this would not be dangerous nor would it conflict with any existing controls.