[Canvas] Prevent tray collapse button from overlaying the side bar content

[ryankeairns]

Kibana version:
7.0.0-alpha

Browser version:
Chrome

Describe the bug:
It's not necessarily a bug, but there is a long bar on top of the tray that blocks some of the sidebar content. The only interactive part of this element is the down arrow icon, the rest of the containing bar is just taking up space.

Steps to reproduce:

1. Select an element on a workpad
2. Open the Expression editor (see bar above expression editor)

Expected behavior:
The icon is in the same position, but the background is only enough to contain the icon, not a full-width bar.

Screenshots (if relevant):

cc:/ @cqliu1