Issue with drag and drop in modern UI

What browser are you using? I tend to see a similar behaviour if I use Internet Explorer, however when I'm using Chrome it works like a dream.

Sometimes I have to give the browser time to catch up, so just have to hold the file over the drag and drop area for a second or so until it realises what I'm trying to do. This is usually if I've got something resource intensive running in the background at the same time as I'm trying to drag and drop.