At last! worked it out. It does not make any sense, but ...
The problem is that the table is inside a <fieldset class="invisiblefieldset">, and invisiblefieldset is set to be display: inline;.
I have fixed this on the quiz editing page by the ugly expedient of changing the HTML to <fieldset class="invisiblefieldset" style="display: block;">.
Testing and QA
1.8 Display Issues
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