The Tool UI displays editors (value editors, toggle buttons or menus) and labels. Visual linking relies on the attribute labels as zones to trigger a link gesture. Labels are already used for marking.
To link the Red and Blue components of a color input in the same group:
This sets an expression on the Blue channel attribute referring to the Red channel attribute without ever displaying the Expresso Calculator.
In the same way, you can link inputs belonging to separate groups, setting an expression in the target attribute referring to the source input.
