Add the sugar, the flour, and the celery seeds, and cook the mixture, stirring, for 30 seconds. Stir in the mustard, the vinegar, and 1/2 cup water, bring the mixture to a boil, stirring, and simmer it for 2 minutes, or until it is thickened. Season the dressing with salt and pepper, pour it over the potatoes, and stir in the eggs, the pickles, the bacon, and the scallion greens.