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 establishes a new block formatting context, its width should be limited by the float. Safari and IE7+ are correct. In Opera 9.5-10.10 the width of the blue box is limited by the float, but the box is displayed below the float as if it cleared the float. The "drop" is probably an acceptable choice, but then why the box is still limited by the float and does not take all the available width? The problem is fixed in Opera 10.50
IE8 and Fx3+ seem to ignore the negative left margin (they render like it was 0).
Same problem in Opera 9.5-10.10 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). Same problem in Opera 9.5-10.10.