Hello,
I'm having the same issue as listed in this [locked] post.
The youtube video that was linked in that post is exactly what I'm seeing. I have a similar setup, also - possibly it's something to do with running this on a macbook, and plugging in an external monitor? I restarted Rstudio and it went away - but it has definitely occurred multiple times. I'll let you know if it happens again, and under what circumstances. FYI, before restarting RStudio I tried the last two items listed in Kevin's response, and neither of them stopped it.
RStudio Version 1.2.1335
Build 1351 (f1ac3452)
MacBook Pro (15-inch, 2019)
2,6 GHz Intel Core i7
32 GB 2400 MHz DDR4
Intel UHD Graphics 630 1536 MB