To make the Xfce Window Buttons applet (or simply the Taskbar, as all of us are used to call it) multi-row, add the following to ~/.config/xfce4/gtkrc (create the file if it doesn’t exist):
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include "/usr/share/themes/greybird/gtk-2.0/gtkrc" # Change 'greybird' to whatever Xfce theme you're using
style "xfce-tasklist-style"
{
XfceTasklist::max-button-length = 192
XfceTasklist::max-button-size = 16 # Make sure that this value is at least two times less than the panel size
XfceTasklist::ellipsize-mode = PANGO_ELLIPSIZE_END
XfceTasklist::minimized-icon-lucency = 50
XfceTasklist::menu-max-width-chars = 24
}
class "XfceTasklist" style "xfce-tasklist-style"
After you relogin, your panel should look like this: