A contentanchor control has been added to this windowclass. It relies on the title control being present, but the Trail of Cthulhu ruleset deletes this windowtitlebar in the PC selection window.
It would be best for cscase to look at this so that the fix fits with the original design goal.
However, a quick fix is to comment out the <windowtitlebar name="title" merge="delete" /> code in the <windowclass name="charselect_host" merge="join"> in campaign\campaign_chars.xml in the Trail of Cthulhu ruleset. The same will have to be done for the title_main and title_local controls in <windowclass name="charselect_client" merge="join"> in the same file - comment out those 2 controls.