Steps to repro:

1. Open Unity;
2. Open attached project and 'test' scene;
3. Select Terrain gameobject in the Hierarchy;
4. Go to 'Paint Height' tab of Inspector's Terrain component;
5. Minimize Inspector window;
6. Notice that scalebar has disappeared - expected behaviour;
7. Hover over zone on the left side of Height textbox and try to change its value by dragging left-clicked mouse cursor.

Actual result:
There is no ability to customize Terrain Height field by dragging mouse cursor slider (see attached screenshot).

Expected result:
User should be able to change Height value in such way, assuming that for other textboxes within Unity (e.g. Transform component) there is an ability to change textbox values by dragging mouse cursor slider.

Notes:
- Reproducible in 5.4.0b21, 5.4.0b20, 5.3.5p1, 5.2.2p4;
- Reproducible on both OSX 10.11.4 and Windows 10.