How difficult would it be to make it so that, when you double-click on the resize bar of a child element, that it goes back to its original position?

Windows has such functionality by default. When viewing a table of information, such as a list of user accounts, it's possible to move your mouse over the table until the resize cursor is shown. Then if you double-click, it will resize that column of the table accordingly to fit the data that resides within.

For child elements, it could either resize the element to its original setting, or resize to fit the contents within (perhaps allow as an option? or maybe right-clicking resizes to original whilst double-clicking resizes to fit the contents)?