Most browsers cannot determine the width of an element if it is not currently visible. That includes the case where one of the parent elements is hidden. That makes sense when you think about it; the layout engine doesn't know how the element would "look" unless it's visible and interacting layout-wise with its parent and sibling elements.
If jQuery's .width() method detects that the element you are trying to measure has display:none, it will temporarily set the element to show so that it can be measured. However, it does not try to apply that rule recursively to all the parent elements up to the body. That would be pretty time consuming. You'll need to defer the width calculation until its parent div (tab) is visible, or temporarily show that tab long enough to do the calculation initially and then switch to the tab selected by the url. I've had a similar problem to this when trying to use my splitter plugin on one tab that isn't initially visible; the splitter needs to know the dimensions of the parent element to initialize.
