Steps to repro:

1. Open Unity;
2. Create new project;
3. Import attached 'TerrainInspectorBug' package and open 'test' scene;
4. Select 'Terrain' gameobject in the Hierarchy;
5. Go to 'Place Trees' tab in Terrain Inspector;
6. Double click on' Broadleaf_Desktop' tree prototype;
7. Close the popup window;
8. In the Hierarchy select any other gameobject (e.g. Main Camera or Directional Light);
9. Hover over terrain.

Expected result:
Terrain brush should not be rendered and should not be active when current Terrain is not selected in the Hierarchy.

Actual result:
Terrain brush is rendered and active even when current terrain is not selected in the Hierarchy. See attached video.
Regression introduced in 5.6.0b4.

Notes:
- Reproducible in 5.6.0b5, 5.6.0b4; not reproducible in 5.6.0b4, 5.6.0b2, 5.6.0b1, 5.5.0p4, 5.4.4p1;
- Reproducible on both OSX and Windows;
- The bug may be related to recent changes in Terrain Inspector, shipped to 5.6;
- Also reproducible after opening other terrain foliage popup windows, like "Add Grass texture" or "Add Detail Mesh".