CSS transition from a fixed px width to an auto width

I was struggling trying to animate the width of a <div> from a fixed px width to a 100% (or auto) width. The idea was to have a completely fluid 2 columns layout. A main col taking all the space and a right col with a fixed width. Then, on a particular user action, the right col animates until taking all the remaining space (overlapping the main col). I needed to achieve the effect only with css transitions.

Here's how I did it...

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