I don't think it's platform-specific. Here are the diffs and frequency of flakiness:
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.