Report/react to layout geometries that won't fit in the availble window

[freakboy3742]

Via #906, it became clear that it is possible to specify layout geometries that exceed the specified size of a window - i.e., you can specify a geometry that requires at least 350px of horizontal space, and also specify a window width of 300px.

In this situation, we should:

1. Report that there is a geometry problem
2. Modify the size of the window so that it is at least the minimum viable size.