What theme, xfce4, and Debian version are you using? It looks like some kind of GTK 3 theme issue I've seen with a backported MATE 1.16 that I did for Jessie--some newer GTK 3 themes don't work very well with an older GTK 3, and vice versa.
The MX Linux repositories
: Backports galore! If we don't have something, just ask and we'll try--we like challenges. New packages: AzPainter 2.0.4, Pale Moon 27.2.1, Liquorix kernel 4.10-2, mpv 0.24, Kodi 17.1, Ksnip 1.3.1, Mesa 13.0.6