How to force an element to fit 100% of the available width, even on window size changes?

I have been working on a project that require an awesome tool called Masonry to align elements through javascript, and here is the thing: I don't know how to force my html document to kind of "recover" the initial ratio when the window is maximized in case that user reduce the window size.

Is not a web browser engine issue, same stuff happen in gecko and webkit.

My trouble in action

Please notice the container width at the beginning and at the end, that slight difference, that annoys me.

I know that is not necessary to apply 100% width to a div element, because a div automatically displays as big as it's parent element (in this case body), but this "bad practice" helped me a little to get a better result. (Previously the issue was a bit worse).

Calling css gurus.

Thanks in advance.