[Mac] [Preview] Texture nodes do not clip behind parts outside of the graph editor until the node is completely obfuscated

When using the Material editor, the preview slides on the texture nodes do not clip behind the other elements of the UI as the nodes do.

This issue only occurs on Mac OSX El Capitan version. It was tested on 10.11.3 but was originally reported in 10.11.4b6

Regression: Yes
Occurs in: 2896623 (4.11 Preview 7)
Does not occur in: 2872498 (4.10.4)

Will narrow down to see if the issue occurred in 4.11 Preview 6 if needed.

1. Open a project in the editor
2. Create and open a new material
3. Add a Texture Sample node
4. Left-click and drag the graph so that the newly added node goes upwards off the screen
   Result: Notice that the preview for the texture sample is over the toolbar until the entire node is off the screen
   Expected: The preview should clip as everything else is.