cua-virtual-rectangle-edges

cua-virtual-rectangle-edges is a variable defined in `cua-base.el'.
Its value is

t

Documentation:
If non-nil, rectangles have virtual straight edges.
Note that although rectangles are always DISPLAYED with straight edges, the
buffer is NOT modified, until you execute a command that actually modifies it.
M-p toggles this feature when a rectangle is active.

You can customize this variable.