Content in a horizontal flexbox (accordion) overflowing

Question

I have a problem with a flexbox (accordion). In the example feedled below I have groups of contents, with each group having a cell in a horizontal flexbox and inside each cell the contents pertaining to each group.

Live example & code here.

The problem is that when the content of a group is too big, it overflows the flexbox and i can't come up with a solution for this.