er.IronFitBehavior`.

### Backdrop

Set the `with-backdrop` attribute to display a backdrop behind the overlay. The backdrop is
appended to `<body>` and is of type `<iron-overlay-backdrop>`. See its doc page for styling
options.

### Limitations

The element is styled to appear on top of other content by setting its `z-index` property. You
must ensure no element has a stacking context with a higher `z-index` than its parent stacking
context. You should place this element as a child of `<body>` whenever poss