The Windows and Windows NT environments use a "background brush" when they resize a window. By default, the background brush color for ACUCOBOL-GT is black. If you have arranged your default background to be white, you will see a black flash when you resize the window. This does not affect the final appearance of the window, but is briefly noticeable while the window is being redrawn.
If you desire, you can set the configuration variable WHITE_FILL to 1 to cause ACUCOBOL-GT's background brush to be set to white instead. Doing this will also cause the initial screen that ACUCOBOL-GT paints to be white instead of black.