However, seconds after I had posted the question, I suddenly noticed that QT's search box was smaller than usual, and I realized it had been so for a while. So I grabbed the right edge, and dragged the box wider, towards the right edge of Explorer. And voila! Explorer made itself wider, to adjust to the search box. I closed & restarted Explorer. Yes, it was good. I closed & restarted Vista, Yes, it is still good. Problem solved, without knowing why.
Sorry for taking up your time, but it seems I sometimes have to form a question, before I can find the answer (sounds quite logic, come to think of it)!