When croppin in "Dim The Lights" or "Lights Out" modes, closing the crop tool while still in the develop module makes the entire screen, including the image, go dim or black, respectively.
System Information:
Lightroom version: CC 2015 [1014445]
License: Creative Cloud
Operating system: Mac OS 10
Version: 10.10 [3]
Application architecture: x64
Logical processor count: 4
Processor speed: 3.2 GHz
Built-in memory: 24,576.0 MB
Graphics: NVIDIA GeForce GT 775M 1024 MB