Repro steps:
1. Open user attached project "OnSceneGUINotCalled"
2. Open scene "SampleScene"
3. Select "Test Object" in the Hierarchy and observe the Scene View and the Console
4. Select "Main Camera" or "Directional Light" in the Hierarchy
5. Lock the Inspector Window
6. Reselect "Test Object" and observe the Scene View and the Console

Expected: Lines are drawn around the game object and "OnSceneGUI called" messages are generated in the Console
Actual: OnSceneGUI isn't called and no lines are drawn

Reproducible with: 2017.1.4f1, 2017.2.3p1, 2017.4.4f1, 2018.1.2f1, 2018.2.0b6

Notes: When you lock the inspector on the "Test Object" and select other game objects the lines are always drawn and OnSceneGUI is called constantly.