Draggable element-Prevent drop if the element is still inside the container div

Question

I've found no answer yet for my problem.

I have a DIV A and a modal DIV M as container which contains draggable boxes. Div M is on top of div A.

I can drag and drop boxes from div M to div A, which is the expected behavior.

My problem is when I start dragging a box and drop it while the box is still entirely in div M, the box will still be dropped in div A, and that is not expected.

How is it possible to force not to drop while the box hasn't left the container div M?

I tried to play with all droppable/z-index settings but no luck.

Although the container div overlays the parent div, it's like it is transparent and it can't prevent any drop action.

Thx for your help.