Info Addon: increase container's z-index

[kohlmannj]

As it turns out, there's a weird interaction between the info addon and some of the custom decorators I'm using:

I have a custom decorator that renders semitransparent placeholder text below the story. When clicking on the blue "Show Info" tab, the Info content shows up, but because it has no background, the placeholder text shows through:

[Update]: ah, this is actually a z-index problem with the fixed-position container that the info addon uses. The placeholder text content has z-index: 2 (for reasons beyond its usage in Storybook).