Currently, when sidebar resizers are dragged towards a div that contains an iframe, the mousemove events are captured by the iframe contentDocument rather than the resizer's one. This is perfectly reproducible on Windows Safari.
Created attachment 33095 [details] patch
Created attachment 33169 [details] patch
This is a bug in Windows Safari I think, since this does not happen on the Mac. The events should bubble up to the main frame if they are not captured and stoped from propagating. We should fix the bug in Windows Safari.
See also: bug 20878.
Can you make a test case that fails outside of Web Inspector?