I would guess from this that you are a Mac user. If so, this is a known issue with certain zoom settings on Mac systems. As far as I know, it doesn't have anything to do with the presence of a container field on your layout:
For More Information see: Edit Layout/Report in Pro 11 Screen turns to gray when using either of the scroll bars.
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Mac's are classically faster graphics cards and "outrun" the FMP standard screen refresh or update. A work around is to force a screen update with a script "Refresh Window" , use the green "+" button twice, or Zoom in and out. Another is to slow down your mouse click and repeat.