I've been knee deep trying to debug this chromium issue http://crbug.com/74826 that I believe is not port specific and may be related to https://bugs.webkit.org/show_bug.cgi?id=49023.
Basically, the timeline slider control is being painted fine and the correct rect is hitting the theme, but mouse down events never reach the control, but instead are fired on it's parent element.
It is as though the RenderSlider always has it's default desired width as far as hittesting is concerned.
For background, the slider and it's parent are flexible boxes. If I remove "-webkit-appearance: media-slide" (changed to none) and the height (changed to auto), the correct behaviour occurs, but if either (or both) of them are changed back, the problem occurs.
Found the bug in RenderSlider.
Closing this bug as it's now been fixed in https://bugs.webkit.org/show_bug.cgi?id=61132