RStudio is not displaying text---sometimes the script, also object names, and packages (see screenshot). The text is there---it hasn't disappeared and sometimes I can highlight the script with my cursor and it appears, then it goes away again. I think this started when I recently updated RStudio and R.
It might help to know your operating system and RStudio version (build 394?). Also, if you go to Tools > Global Options... > Appearance, does changing either the RStudio theme setting or the Editor theme setting fix the issue?