Just fired up TDM on new system after a pause, built dark radiant via ebuild from wiki install instructions and encountered this bug. Small texture preview in "media" tab is not updating, and so is "textures" tab. Texture selector in Surface inspector works fine however. Textures tab updates and show textures when typing into filter field.
I also get a lot of spam in terminal, in shape of "(darkradiant:10678): Gtk-CRITICAL **: gtk_box_gadget_distribute: assertion 'size >= 0' failed in GtkSpinButton", but that may be unrelated.
I upgraded wxGTK to latest (3.0.4) and then checked out and built last tagged version (2.6.0) of radiant, to no avail.