When the user presses ESC, currently the framebuffer window closes, yet the rendering proceeds. This behavior is somewhat unintuitive.
If a user presses ESC, it is likely his intentions are to abort the current operation: in this case, cancelling the render.
Nearly all 3D packages (and other commercial renderers) behave in a similar way when a user cancels a render (by pressing ESC). They immediately abort the rendering process, and present the user with the contents of the frame buffer up to the point of cancellation. I.e. the user can review the intermediate render, easily stopping it without waiting for it to completem, in an elegant and intuitive manner.
In summary, the framebuffer could be improved if the ESC key was handled differently to what it does now: rather than merely closing the window, abort the render and present the user with the incomplete render, as in other packages.