Eclipse - can't delete discovery tiles

Playing our first game of Eclipse on Vassal/PBEM. After exploring/discovering a new sector and drawing a discovery tile for it, I can right-click on the discovery tile to “use it for effect,” and the message shows up correctly in the logging, but…

After that the tile is to be discarded, but I can’t find a way in the module to discard it. No right-click menu option, key on keyboard doesn’t do it. What am I overlooking?