React Markdown App, need advice to keep output on white background

Please take a quick look at this webpage:
Notice how the output extends beyond the white box labelled Preview. I would like the Preview output to always be displayed against a white background. I created a div and gave it a size and border so that the output would be in it. But it appears the innerHTML is not bound by the div that surrounds it.

I would either like to make my preview box height match the output size, or I would like the output to be kept inside the current box with scrollbars.

Can someone suggest how I can do either of these? Thanks.

Add this to your stylesheet, this will add scrollbars on your Preview if there’s overflow.

.preview {
 overflow: auto;

Thank you. That one css property is exactly what I needed.