Building from the latest sources on the Cog branch, I see the attached image when loading various images using various OSX builds. I have tried the 64-bit Pharo, Squeak and newspeak builds with their associated images. I see some variation of the attached image below. If I rollback the source to the 201901172323 tag, I am able to build and load an image and see the window as expected.
<img width="811" alt="Screen Shot 2019-04-23 at 6 47 51 PM" src="https://user-images.githubusercontent.com/32800969/56627814-3fea6280-65fc-11e9-8bb0-e836cbf44187.png">