`sleep(100)` will not work. The correct version would be to pop open your window and return to the main call, and then react to events as they happen later. If you want the window to close after a certain time, even if the user did not click a button, you can create a timed callback with `Fl::add_timeout()`, but you must make sure to remove the timeout if the user closes the window in the conventional way.
It's not good style, but you can also busy-wait until the dialog is closed, something like:
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