Fix #1955 (expandable): Left panel features coexistance

Changed so that the width of the left container is calculated depending on all columns in it. Without Math.floor() the left container takes too much space when left pinning a column (thus pushing the body beneath it), I guess this has something to do with rounding issues elsewhere. I also included a second example in expandable tutorial where the coexistance of these features that utilizes the left renderContainer can be verified.