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### Overlay

An overlay is used when it is desirable for a block of content to be shared between several different windows. In addition, it can be used to append or alter content in an existing window. An overlay is defined in a separate XUL file.

Each element within the overlay is inserted at a location in the parent window, determined by matching id attributes. For instance, if an element in an overlay has an id of 'filemenu', the corresponding element with the id 'filemenu' in a window that uses the overlay would be altered. Attributes declared in the overlay are added to that element and child elements are inserted to the window within that element.

Overlays may be applied to windows in other packages. Files, such as scripts, that are referenced from the overlay should be specified as absolute URLs. If you do specify a relative URL, it will relative to the window that the overlay applies to, not the overlay's file.

More information about Overlay

#### Attributes:

class

The style class of the overlay. Multiple classes may be specified by separating them with spaces.

id

A unique identifier so that you can identify the overlay with. You can use this as a parameter to getElementById and other DOM functions and to reference the element in style sheets.

#### Properties and Methods:

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