The Mysterious White Gap
You have the Trackbar floating to the right in an area that is 3px larger than the trackbar itself. If the trackbar needs to be 17px, reduce the 20px margin for the outer container and corresponding -20px margin for the trackbar to 17px and -17px, respectively.
The parent div is 310px, the div to the left is only 290px, but your trackbar is only 17px (instead of the remaining 20px).
Alternate Solution
An alternate solution would be to provide a background color for the parent div so the blue color also goes behind the trackbar.
Debugging CSS/Positioning
A handy way to debug issues like this is to use Chrome's debugger. Press F12
in Chrome and select the little magnifying glass icon in the bottom left corner. You can move the mouse over gaps on the page and see what is creating that space.
You can also look at the Computed Style section on the right side of the Elements tab to see exactly what values a DOM node has for each property and what is causing/influencing those properties.
Firebug has an Inspect function which should behave similarly.