Strange behaviour of custom drag and drop function - vuejs2

Question

I'm currently creating an electron application (Frontend is in Vue) which can trigger native Drag-and-Drop-Operations. These Operations behave strange if i trigger them with HTML5 Drag Events (like dragstart, dragstop, etc.). So I have to implement my own Drag-and-Drop Handler using mouseup, mousedown and mousemove. So in simple theory:
If MouseDown is triggered -> Set Flag
If MouseUp is triggered -> Remove Flag
If MouseMove AND Flag -> checkCurrentPosition vs. initialPosition -> Do something
new Vue({
el: "#app",
data: {
mousedown: false
},
methods: {
onMouseDown: function(e){
console.log('mousedown');
this.mousedown = true;
},
onMouseUp: function(e){
console.log('mouseup');
this.mousedown = false;
},
onMouseMove: function(e){
if(this.mousedown) {
console.log('mousedown && mousemove');
console.log('clientX:', e.clientX);
console.log('clientY:', e.clientY);
}
},
}
})
Fiddle
if you check this example, open up your browsers console and play a little bit around with the image (click, drag and drop,...) you will see that if you drag the placeholder image, the mousedown event is triggered and the flag is set. Then I would expect that the mousemove event would be triggered frequently, but this is not happening.
Normally if you bind mousemove to some Div its triggered frequently if you hover with your mouse over it and you can get the actual position for example. But this is not happening after mousedown.
NOTE: The mouseup event in the Fiddle is not bound to Window, so if you Drop the Element outside of its area the mouseup event will not be triggered for the element. So the flag is not removed and then you can see the behaviour I would expect. the mousemove event is frequently triggered. So I think that the problem is that while the mouse is pressed the mousemove event is not triggered correctly.