I didn't find an easy way to see class definitions with the Geant4 LXR (sorry if I overlooked at something). For example if I am at the page http://www-geant4.kek.jp/lxr/ident?i=G4CascadParticle and I click at the link "class G4CascadParticle" I am redirected to the same page. The only way I figured out was to go to the file where the constructor is defined, then identify the header file where the class could be defined (not very difficult in this case but it can be in general) and open the header file and browse it to find the class definition.
I understand what you want. Sorry, but it is a normal behavior of LXR, not a bug. Instead of identifier search, use file search for a quick way of reaching class definition, because most of Geant4 classes are defined in files whose file names are class names.