Dropzone blue line indicators not appearing consistently

[chriskmnds]

Issue Overview

There is some inconsistency with the editor dropzone indicators: When dragging an image over the blocks, you sometimes need to pass over the top/bottom ends of the editor twice for the blue line indicators at the top or bottom to appear. This is not related to speed - the situation is the same whether dragging slowly or fast.

Steps to Reproduce (for bugs)

1. Create a post with a couple of blocks.
2. Grab an image from the desktop and drag over the blocks list top to bottom a couple of times (see gif below).

Borwsers: IE11, Firefox, Chrome, Safari.

Expected Behavior

The top/bottom horizontal blue lines should always appear when dragging an image over the top/bottom blocks.

Current Behavior

The top/bottom blue horizontal lines either appear randomly or when the dragged image passes over them twice.

Screenshots / Video

This happens regardless of drag speed (the gif above is somewhat misleading in that regard).

[jeffpaul]

This ticket was mentioned in Slack in #core-editor by jeffpaul. View the logs.