Connections signal handler is written as parametrized function and is misidentified by the Connections List as "Empty", even though Connections Editor Form identifies it correctly as "Custom".

Regular "custom" function is correctly identified by the list as "custom" function.

Code snippet (add it to mouse area): 

Connections {
    target: mouseArea    
    onPositionChanged: mouse => {
        if (mouseArea.pressed) {
            const hDelta = mouse.x - mouseArea.oldX
            const vDelta = mouse.y - mouseArea.oldY
        }
        mouseArea.oldX = mouse.x
        mouseArea.oldY = mouse.y
    }
} 

Enclosing parameneter in parenthesis helps to identify the Action as "Custom"

onPositionChanged: (mouse) => {
    ...
} 

but saving file instantly reformats it and removes the parenthesis and marks action as "Empty".