How to expand the panel width to 70% of browser width

I need to know whether its possible to adjust the box(panel) to span 70% of the browser width when clicked or selected and show half of two boxes on each side(left and right) with the arrow mark in it
like this:
leftarrow with partial box] [---------------selected panel-------------------] [rightarrow with partial box

When you click on the arrow it should then show the next box expanded to 70%
