The drag and drop specifications require the "dataTransfer"'s object "dropEffect" attribute to have certain predetermined values on "dragEnter", "dragOver" and "dragLeave". Drag enter and drag over should have a "dropEffect" based on the current "effectAllowed", and "dragLeave" should always have a "none" "dropEffect". Currently, Chromium doesn't adhere to these rules. With the launch of this feature, Chromium will start respecting the specification and assigning the correct values to this attribute so that web developers can start relying on it.
Many bug reports have been filed over the years by developers being affected by the lack of a correct dropEffect implementation: https://issues.chromium.org/issues?q=title:*dropEffect*