what is the wnck-applet?

Solution 1:

From my understanding wnck-applet in MATE is legacy of GNOME Panel, before GNOME Shell introducion.

Some clues from Debian codesearch:

• libwnck3/40.0-2/HACKING:

  gnome-panel/applets/wncklet/
    The actual panel applets based on libwnck (the window list, window
    selector, workspace switcher and show desktop button) are found in
    this directory under gnome-panel, and all run as the process
   "wnck-applet".
  

• org.mate.panel.Wncklet.mate-panel-applet.desktop.in.in

  ...
  [WindowMenuApplet]
  Name=Window Selector
  Description=Switch between open windows using a menu
  ...
  MateComponentId=OAFIID:MATE_WindowMenuApplet;
  ...
  X-MATE-Bugzilla-OtherBinaries=wnck-applet
  
  [WorkspaceSwitcherApplet]
  Name=Workspace Switcher
  Description=Switch between workspaces
  ...
  MateComponentId=OAFIID:MATE_WorkspaceSwitcherApplet;OAFIID:MATE_PagerApplet;
  X-MATE-Bugzilla-OtherBinaries=wnck-applet
  
  [WindowListApplet]
  Name=Window List
  Description=Switch between open windows using buttons
  ...
  MateComponentId=OAFIID:MATE_TasklistApplet;OAFIID:MATE_WindowListApplet;
  ...
  X-MATE-Bugzilla-OtherBinaries=wnck-applet
  
  [ShowDesktopApplet]
  Name=Show Desktop
  Description=Hide application windows and show the desktop
  ...
  MateComponentId=OAFIID:MATE_ShowDesktopApplet;
  ...
  X-MATE-Bugzilla-OtherBinaries=wnck-applet
  

In Ubuntu mate-panel package depends on libwnck-3-0 package, so above assumptions are actual.

As an experiment you can try to run killall wnck-applet on the system with MATE desktop using default settings. It will lead to crashes of the above mentioned applets on the bottom MATE Panel - Show Desktop, Window List and Workspace Switcher (and Window Selector if you have enabled it). Clicking Reload in each Error window will reload wnck-applet and all the functionality will be restored.