Prevent flapping animations when removing / adding dragover class

Problem: When starting a CSS3 transition which modifies position/z-index/opacity of sub elements of the dropzone after the dragover class is applied, the animation sometimes starts to flap awkwardly.

Solution: Wait 200ms before removing dragover class to prevent issues with CSS3
transitions which rely on the dragover class to be induced (when the dragover class is applied/removed/applied again in short time).