Translucent objects reveal windows behind the OF window

This seems to happen on macOS and Windows (had this issue before on Win10, cannot test it right now).

To trigger this, it is sufficient to set a mesh's diffuse color partially transparent. It happens with and without the "Blending" checkbox checked. I am using the "Default internal renderer" on macOS.

Using apitrace, I noticed some glClearColor calls with 0 alpha channel, but was unable to track down their origin.