Steps to repro:

1. Open attached project and 't1' scene on MacBook with the integrated Intel Iris GPU;
2. Open the Lighting window (Window>Lighting>Settings);
3. Select 'Cube' gameobject in the Hierarchy;
4. In the Lighting window go to the 'Object Maps' tab;
5. In the Preview panel of the 'Object Maps' tab expand the dropdown list and select 'Baked Lightmap' option;
6. In the Preview panel zoom in by scrolling the mouse wheel.

Expected result:
Baked Lightmap texture is clipped into the borders of the Preview panel.

Actual result:
Baked Lightmap texture takes all space of the Lighting window. See attached video.

Notes:
- Reproducible only in OSX on a device with the integrated Intel Iris GPU; may not be reproducible on other Macs with the discrete GPU (NVIDIA/AMD);
- Not reproducible on Windows 10;
- Reproducible in 2018.2.0a1, 2018.1.0b5, 2017.3.0p2.