You should always have one event loop live for your application. If you make two windows in sequence, the second call to loop (or simpledisplay's SimpleWindow.eventLoop, upon which this is built) might fail:

// don't do this!
auto window = new Window();
window.loop();

// or new Window or new MainWindow, all the same
auto window2 = new SimpleWindow();
window2.eventLoop(0); // problematic! might crash

simpledisplay's current implementation assumes that final cleanup is done when the event loop refcount reaches zero. So after the first eventLoop returns, when there isn't already another one active, it assumes the program will exit soon and cleans up.

This is arguably a bug that it doesn't reinitialize, and I'll probably change it eventually, but in the mean time, there's an easy solution:

// do this
EventLoop mainEventLoop = EventLoop.get; // just add this line

auto window = new Window();
window.loop();

// or any other type of Window etc.
auto window2 = new Window();
window2.loop(); // perfectly fine since mainEventLoop still alive

By adding a top-level reference to the event loop, it ensures the final cleanup is not performed until it goes out of scope too, letting the individual window loops work without trouble despite the bug.