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:
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