Using the latest Firefox. However, have also tried with Chrome, Waterfox (old Firefox fork) and Opera, with the same unacceptable result. There also appears to be a slight misunderstanding - it's not the fact that the tabs default to 'top', but the fact that the Canvas and Legend disappear on the ostensible positioning of the tabs at the top of the page, which appears to be achieved by the 'centred' automatic vertical scrolling of the browser.
This behaviour is also not consistent, however - if you have a look at the attached images dumped while
using Opera , screen 1 (first tab) shows a complete Canvas image, screens 2 and 3 (second and third tabs) show no Canvas or Legend, and screen 4 (fourth tab) shows the Legend and a partial Canvas image. What is being displayed is clearly currently dependent upon the number of fields attached to the tab concerned, with the focus being set the minute the Tabs are displayed at 'page top'. Can this not be changed so that the Canvas, Legend and Tabs remain displayed at all times as 'page top', irrespective of which tab has the current focus or how many fields it has?
Awaiting your further advices - many thanks indeed!