Spinner: Unit tests failure in Firefox after mousewheel update

This is a weird one. Since updating the mousewheel plugin, spinner unit tests have been failing in Firefox 27 and 28, for example this: http://swarm.jquery.org/result/1778279 (the test uses

equal

with wrong argument order, the correct result is

"2"

).

Reverting that commit or removing the mousewheel plugin from the spinner test suite works around this issue - the test involving mousewheel doesn't actually need the mouswheel plugin.

I tried to figure out why this test is failing. I found that the problem disappears when removing these two lines (both were introduced with the plugin update): https://github.com/jquery/jquery-ui/blob/919d9185f2cf586f794b367a2570368fd81f3879/external/jquery.mousewheel.js#L46-47

Removing just one of them isn't enough - as long as one of these lines exists, the problem occurs.

Inside of these methods, calling

$(elem).height()

or

$(elem).offsetParent()

seems to be responsible. Calling

.parent()

works fine. I ran through the

offsetParent

call with the debugger and there's nothing suspicious happening. I still have no clue why or how that effects this spinner unit test where we only a cursor UP event and check the value.

I currently tend to just removing the mouswheel plugin from the spinner test suite. Before doing that I wanted to see if someone has an idea what might be going on here.