If you apply that one, could you please add this too?
In themes/default/css/index.css
.topic_sorting #sortby, #sortby,
.topic_sorting:hover #sortby, #sortby:hover {
.topic_sorting #sortby {
width: 8.4em;
}
.topic_sorting:hover #sortby, #sortby:hover {
Sorry for the trouble...
ETA: it's not perfect (because there is still a slight "jump"), though it's better than before...