floater
margin-left:-200px; overflow:auto;

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. In Opera 9 (and 8.5) it is 200px shorter: it seems that the width is computed before applying the negative left margin. Firefox 1.5, Safari 1.3.2, IE7, iCab 3.03 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.)

floater
margin-left:-200px; overflow:hidden;

Same problem in Opera with overflow:hidden.

floater
margin-left:0; overflow:auto;

No problems in Opera without negative left margin.

floater
margin-left:-200px; overflow:visible;

No problems in Opera with overflow:visible.

floater
margin-left:-200px; display:table; blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah

No problems in Opera with display: table (filled with content to counteract the shrink-wrap effect.)

floater
margin-left:-200px; display:inline-block; blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah

With display: inline-block. More browsers' differences...

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