I thought that field is for changing the target display size in order to test your design against various viewer screen sizes. As such, I change that setting all the time. In fact, when you drag the sizing handles on either side of the working page, that setting changes automatically. However, that does not fix my issue.
Upon further experimentation I have discovered that the problem only occurs when you HAVE used CTRL± to reduce the workspace size. But if you want to test your webpage for a large screen (by setting the value circled in red in iamteejay’s comment to a large value) then the workspace display always extends off the screen. When that happens you cannot view and fix problems you cannot see. So it seems you either need a way to manually scroll to examine all parts of the webpage or use CTRL±, the latter of which seems to have a bug.
Has anyone else experienced this?