The body has a 200px left margin, the green box is right floated, the blue one has a -200px left margin and overflow:auto. The blue box should stretch horizontally till the float. Firefox, Safari, IE7 are all correct (IE6 is wrong, overflow:auto does not establish a "block formatting context" and the blue div stretches to the full page width.) In Opera 9 (and 8.5) it is 200px shorter: it seems that the width is computed before applying the negative left margin. In Opera 9.5 the width of the blue box is correct, but it is displayed below the float as if it cleared the float (probably not wrong, but not necessary.)
Same problem in Opera with overflow:hidden.
No problems in Opera without negative left margin.
No problems in Opera with overflow:visible.
With display: table (filled with content to counteract the shrink-wrap effect). No width problem in Opera 9, but still the clearing effect in Opera 9.5.