This effect is some annoying bug in OpenMotif and its management of the neighboring widgets that make up the threshold slider and the colorscale bar. I have tried to patch over this problem several times, but for some reason the patch started causing trouble with a recent AFNI build. I simplified the patch, as Paul described above, and Rick did an emergency build Monday night to push this change out. The downside is that at some point you will have to manually fix the threshold slider size by left-clicking on "Thr".
I will continue to try to make this problem less visible, but it has bothered me for years, and I'm about ready to scream.
