Web ListBox renders taller than its grid area and covers the gap below it

In a running web app, a ListBox can render taller than the grid area it occupies on the page, covering the gap that separates it from the control below. The page designer shows the correct spacing — one grid gap between vertically adjacent controls — so the running page and the designer disagree: ListBoxes that are clearly separated in the designer appear to touch in the browser. The ListBox should fill its grid area exactly, like every other web control, so the running page matches the designer.