I have a problem with the Drag & Drop window that appears when you drag a document to a library inside a form.
In my case I have two tabs, for example tab 1 has "lib1" as webpart and tab 2 has "lib2" as webpart.
When you drag a document over the library webpart in one tab, the drag & drop window appears. When you change the tab and drag a document over library webpart the window does not appear.
After looking at the html code I see that the window appears but on tab where I uploaded previous document.
Sometimes the window appears somewhere on the screen but I fixed it by adding custom css for #ms-dnd-dropbox