To determine whether the video mode change callback function is working properly, log in to the system and use fbconfig(1M) to change the resolution and depth of the frame buffer several times. If the console continues to display text properly, the video mode change callback function is working correctly. The kernel terminal emulator might adjust the font size to accommodate different screen sizes, but that is not significant to the console frame buffer driver.
To determine whether the X Window System and the console frame buffer driver interact correctly, switch between the X Window System and the command line several times while modifying the X Window System's video resolution and the command line resolution in different ways. If the X Window System exits and the console characters are not displayed correctly, either the X Window System did not notify the driver console code that the video mode changed or the driver did not call the kernel terminal emulator's video mode change callback function.