I've noticed a bug for a few years (I'm in 1.1.414 now) that breaks my Pane Layout.
Try putting Source in the top left, Console in the top right, and all the other panes but x in the bottom right click OK and it automatically shifts the panes around and you have to go back in and fix everything.
This sounds like a long-standing bug where attempts to move the Viewer tab to a different quadrant fail. Can you confirm if you're seeing the same thing? (Try changing the layout but keeping Viewer at the bottom right)
Yep, that's the problem.
Thanks! Looks like this has been reported at https://github.com/rstudio/rstudio/issues/2101 as well.
The ticket is closed 