moreMenu not working with Vivaldi or RStudio on macOS Catalina

**System info:**
macOS 10.15.5
Vivaldi 3.0.1874.38
RStudio 1.3.959

**Issue:**
+ Vivaldi: The moreMenu arrow is not visible.
+ RStudio: The moreMenu arrow either
  + is not visible, or
  + is visible and clickable, but makes all menus disappear except for "RStudio" and "Window" (other menus won't reappear before rebooting RStudio).