Trying to paint or sculpt terrain draws a selection box instead.

Repro steps:
1. Open Terrain scene from attached project or create a new project and add a terrain.
2. Select terrain, click on Raise/Lower terrain and try painting elevation in the scene view.
3. Observe how instead of affecting the terrain, a rectangular selection box appears.

Expected outcome: to be able to interact with the terrain.

Notes:
- tried on: Version 5.5.0a1 (7540d5fb8ce9)/ Mon, 15 Feb 2016 09:22:18 GMT / Branch: trunk
- regression in trunk; worked in 5.4.0b6
- same behavior appears when trying to pain textures or details.