README.md
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15[](https://travis-ci.org/PolymerElements/iron-overlay-behavior)
16
17_[Demo and API docs](https://elements.polymer-project.org/elements/iron-overlay-behavior)_
18
19
20##Polymer.IronOverlayBehavior
21
22Use `Polymer.IronOverlayBehavior` to implement an element that can be hidden or shown, and displays
23on top of other content. It includes an optional backdrop, and can be used to implement a variety
24of UI controls including dialogs and drop downs. Multiple overlays may be displayed at once.
25
26See the [demo source code](https://github.com/PolymerElements/iron-overlay-behavior/blob/master/demo/simple-overlay.html)
27for an example.
28
29### Closing and canceling
30
31An overlay may be hidden by closing or canceling. The difference between close and cancel is user
32intent. Closing generally implies that the user acknowledged the content on the overlay. By default,
33it will cancel whenever the user taps outside it or presses the escape key. This behavior is
34configurable with the `no-cancel-on-esc-key` and the `no-cancel-on-outside-click` properties.
35`close()` should be called explicitly by the implementer when the user interacts with a control
36in the overlay element. When the dialog is canceled, the overlay fires an 'iron-overlay-canceled'
37event. Call `preventDefault` on this event to prevent the overlay from closing.
38
39### Positioning
40
41By default the element is sized and positioned to fit and centered inside the window. You can
42position and size it manually using CSS. See `Polymer.IronFitBehavior`.
43
44### Backdrop
45
46Set the `with-backdrop` attribute to display a backdrop behind the overlay. The backdrop is
47appended to `<body>` and is of type `<iron-overlay-backdrop>`. See its doc page for styling
48options.
49
50In addition, `with-backdrop` will wrap the focus within the content in the light DOM.
51Override the [`_focusableNodes` getter](#Polymer.IronOverlayBehavior:property-_focusableNodes)
52to achieve a different behavior.
53
54### Limitations
55
56The element is styled to appear on top of other content by setting its `z-index` property. You
57must ensure no element has a stacking context with a higher `z-index` than its parent stacking
58context. You should place this element as a child of `<body>` whenever possible.
59
60
61
62##<iron-overlay-backdrop>
63
64`iron-overlay-backdrop` is a backdrop used by `Polymer.IronOverlayBehavior`. It should be a
65singleton.
66
67### Styling
68
69The following custom properties and mixins are available for styling.
70
71| Custom property | Description | Default |
72| --- | --- | --- |
73| `--iron-overlay-backdrop-background-color` | Backdrop background color | #000 |
74| `--iron-overlay-backdrop-opacity` | Backdrop opacity | 0.6 |
75| `--iron-overlay-backdrop` | Mixin applied to `iron-overlay-backdrop`. | {} |
76| `--iron-overlay-backdrop-opened` | Mixin applied to `iron-overlay-backdrop` when it is displayed | {} |
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