Firefox is not displaying image in drag and drop ghost preview

Question

I'm trying to display ghost element instead of default browser preview for drag and drop. The problem is that in firefox image inside ghost element is not displayed while dragging. But if I drop it, and drag again the image is displayed.

So I think that this might be some sort of cache-related problem. But I can't see how I can pre-cache image in this case.

Here's the code:

//html:

<div class="parent container">
<img class="element" src="http://www.thekrausemouse.com/wp-content/uploads/2016/03/Sample.jpg" draggable="true" />
</div>

//js:

document.querySelector(".element").addEventListener("dragstart", function(e) {
    var img = document.createElement("img");
    var div = document.createElement('div');
    div.style.width = '100px';
    div.style.height = '100px';
    div.style.position = 'fixed';
    div.style.top = '-1000000px';
    div.style.left = '-1000000px';
    div.style.border = '2px solid red';

    img.src = "http://www.thekrausemouse.com/wp-content/uploads/2016/03/Sample.jpg";
    img.style.width = '100px';
    img.style.height = '100px';
    div.appendChild(img);
    document.body.appendChild(div);
    e.dataTransfer.setData('text/plain', 'test');
    e.dataTransfer.setDragImage(div, 0, 0);
}, false);

Fiddle: https://jsfiddle.net/etseq5cg/5/

Steps to reproduce:

1) open fiddle/run snippet

2) try to drag sample image

Actual: you'll see an empty square with red border

Expected: square with image inside.

To reproduce it again you need to force-reload the page(ctrl+f5). That's why I think this is cache-ralated issue.

Note: I know that I should remove ghost element from DOM in dragend handler, but this is not important here.

Update:

1) the actual use-case includes view with big amount of images(~500), so it's not an option to pre-cache images via js.

2) For the ones who couldn't reproduce the issue, here's the screenshot: at first you see preview after hard reload(ctrl+f5), and then the second dragging attempt. Please note that no http requests are seen in network tab in web inspector in both cases.

Answer 1

I cannot see the problem when I run your jsfiddle in Firefox 53 (on Windows 7). The ghost image and the dragged image have the same URL and the ghost image is always displayed when dragging. However, I can reproduce the problem with a ghost image that has a different URL.

You could add a hidden img control to preload the ghost image. Something like this:

<div class="parent container">
    <img class="element" draggable="true" src="http://the.element.image" />
    <img class="imgGhost" src="http://the.ghost.image" />
</div>

According to my tests, these settings prevent the image preload in Firefox:

• Hiding the element with display: none
• Setting a null size (width: 0px or height: 0px)
• Moving it outside of the viewport (e.g. left: -10000px)

I also did not have much success with link prefetching. However, visibility: hidden seems to work. The style of the hidden image element could be defined as:

.imgGhost {
    position: absolute;
    left: 0px;
    top: 0px;
    visibility: hidden;
}

The method can be tested for two draggable images in this jsfiddle. In the dragstart event handler, the image URL is retrieved from the hidden element:

img.src = this.parentNode.querySelector(".imgGhost").src;

but it could be hard coded. If you prefer, you could set the src attribute of the hidden image dynamically when the page is loaded. When testing in the jsfiddle, you can change the ghost image names (e.g. 225x225) before running it again, to make sure that the image was not cached.

---

According to your comment, preloading the image is not an option. And you are using the same image URL for the dragging ghost image. In that case, you could check this page to see if any option is preventing the reload of the image.

You could also force a repaint of the layout after adding the div control to the body in the dragstart event handler. This can be achieved by calling div.offsetHeight:

div.appendChild(img);
document.body.appendChild(div);
div.offsetHeight; // Force repaint