System Information
Windows 10 current Nvidia 1060
Blender Version
Broken:
2.80 current buildbot as well as self-built current master (2.80)
Short description of error
Performing an F12 render with the Preferences window open will re-purpose the Preferences window for the render output.
Exact steps for others to reproduce the error
Start Blender.
Edit->Preferences to open the Preferences (creates a new window)
Click back on the main Blender Window
Press F12 to initiate a render.
Observe that it takes the Preferences Window, expands it to full(ish) screen and changes it to a Blender Render titled image viewer in which the render is displayed rather than actually creating a new window.