Tootlip is essential config when "grouped windows" is used in the panel, currently if compton is enabled tooltip are very transparent (opacity 0) and in compton-conf there is no option to modify this, it should be done directly in the file ~/.config/compton.conf:

wintypes : 
{
  tooltip : 
  {
    fade = true;
    shadow = false;
    opacity = 1;
    focus = true;
  };
};

The idea is to include this configuration option in compton-conf.
Here is upstream https://github.com/lxqt/compton-conf
Here is the issue https://github.com/lxqt/compton-conf/issues/145