Draggable toolbar header class, inheriting from a draggable parent class
I thought I just leave a general review if you don't mind.

MDN has this to say about __proto__:


  Warning: While Object.prototype.__proto__ is supported today in most browsers, its existence and exact behavior has only been standardized in the ECMAScript 6 specification as a legacy feature to ensure compatibility for web browsers. For better support, it is recommended that only Object.getPrototypeOf() be used instead.


The names of some parameters are somewhat short.

function(eH)
function(ev)
function(cnv)


The members of the class that they are modifiying however, have long and descriptive names:

var eventHandler 
var canvas


Classes are made to be reused. As a user of your class, I don't really care about how you name your internal variables or if they even exist. But I'd be really happy to see some readable self documenting method parameter names show up in my auto completion.

setPosition = function(dX, dY)


Imo, if I want to set the position, I'd expect the parameters to be x and y. If they start with a d I'd expect some difference in position as the behaviour of the function. The method name implies an absolute position, but the parameter names imply a relative one.