On my MacBook running macOS 10.13.6, when RStudio preview 1.2.981 is started on my external monitor, is shows up like this:
Zooming in or out only affects the tiny IDE-in-the-window. When I drag the window to the MacBook's internal screen, it looks ok again. It remains normal-looking when I then drag it back to the external monitor. This problem not occur with RStudio preview 1.2.907.
The internal display is a 2560x1600 13" Retina display, while the external monitor is a 3780x2160 43" screen (non-Retina).