For disable Drag and Drop functionality add below code in "Script Editor".

Goto Setting -> Edit Page -> Insert -> Web Part -> Media and Content -> Script Editor and refresh the library.

<script> ExecuteOrDelayUntilScriptLoaded(function() { g_uploadType = DragDropMode.NOTSUPPORTED; SPDragDropManager.DragDropMode = DragDropMode.NOTSUPPORTED; }, "DragDrop.js"); </script>

Thats definitely works. I had tried.

For disable Drag and Drop functionality add below code in "Script Editor".

Go to Setting -> Edit Page -> Insert -> Web Part -> Media and Content -> Script Editor and refresh the library.

<script> ExecuteOrDelayUntilScriptLoaded(function() { g_uploadType = DragDropMode.NOTSUPPORTED; SPDragDropManager.DragDropMode = DragDropMode.NOTSUPPORTED; }, "DragDrop.js"); </script> 

That's definitely works. I had tried.