Displaying an embedded workspace causes field highlights to stop working

[howettl]

Expected Behavior

Field highlights should always work.

Actual Behavior

After displaying an embedded workspace (for example, the custom procedures modal) and subsequently dismissing it, fields will disappear when dragging a compatible block over them instead of glowing.

Steps to Reproduce

Prerequisites: Safari 11.1 or iOS browser (Chrome 66 is not affected).

1. Load the Scratch Preview
2. Add a block to the workspace and verify that the field highlights correctly when you drag a block into it.
3. Click "Make a block" under "My Blocks"
4. Dismiss the modal
5. Drag a parameter into the field again. Notice that the field disappears instead of highlighting.

Operating System and Browser

Mac OS 10.13.4, Safari 11.1
iOS 11