I don't think it's platform-specific. Here are the diffs and frequency of flakiness: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#showExpectations=true&showLargeExpectations=true&tests=fast%2Fmulticol%2Ffloat-multicol.html
It seems that horizontal scroll bar appears when this test is failing. I tested the failing HTML with Mac Safari and Mac Chrome. I randomly changed the size of window and I didn't see any horizontal scroll bars. On the other hand, Linux chrome has a horizontal scroll bar when the width of window is small. I think the behavior of Linux chrome isn't correct.
Seems stable now, rebaselining and closing.