Lock screen UI position is ~32 pixels too high, only after waking from sleep

When the lock screen activates when I wake my laptop from sleep, the UI is about 32 pixels too high (slightly more than my 24px panel on the bottom of the screen). The mouse works like normal (goes to and only goes to the four edges of the screen), so I don't think the X display is in the wrong position. Unfortunately, I can't provide a screenshot of my lock screen, but it looks about like what you would imagine: a black bar along the bottom of the screen with everything offset upwards from where it should be. This does not happen when locking the screen without putting the computer to sleep.

Some info:

• XFCE version: 4.20
• xfce4-screensaver version: 4.18.4
• PC specs:
  • CPU: Intel Core i5 520M
  • GPU: Intel Graphics Media Accelerator HD (integrated)
  • RAM: 4GB DDR3 at 1334 MT/s
  • HDD: 128GB SATA II SSD
  • Display: 1280x800, connected via internal LVDS