if chatting window is open, there will be strange shadow on the screen in debian buster

when i click close button of the right-top chatting window, the strange shadow disappeared.

os:

mmc@debian ~ $ screenfetch
         _,met$$$$$gg.           xxx@debian
      ,g$$$$$$$$$$$$$$$P.        OS: Debian 10 buster
    ,g$$P""       """Y$$.".      Kernel: x86_64 Linux 4.19.0-8-amd64
   ,$$P'              `$$$.      Uptime: 1h 15m
  ',$$P       ,ggs.     `$$b:    Packages: 1672
  `d$$'     ,$P"'   .    $$$     Shell: zsh 5.7.1
   $$P      d$'     ,    $$P     Resolution: 1366x768
   $$:      $$.   -    ,d$$'     DE: XFCE
   $$\;      Y$b._   _,d$P'      WM: Xfwm4
   Y$$.    `.`"Y$$$$P"'          WM Theme: JacWin10
   `$$b      "-.__               GTK Theme: Windows10 [GTK2]
    `Y$$                         Icon Theme: Faenza
     `Y$$.                       Font: 文泉驿等宽微米黑 10
       `$$b.                     CPU: Intel Core i5-3320M @ 4x 3.3GHz [54.0°C]
         `Y$$b.                  GPU: intel
            `"Y$b._              RAM: 5093MiB / 15741MiB
                `""""