XUL:Attribute:state

state: Type: one of the values below; Indicates whether the splitter has collapsed content or not. This attribute will be updated automatically as the splitter is moved, and is generally used in a stylesheet to apply a different appearance for each state.

open: The content either before or after the splitter, depending on the value of the collapsed attribute, is currently displayed.
collapsed: The content either before or after the splitter is collapsed and is not visible.
dragging: The user is current adjusting the position of the splitter, typically by dragging it with the mouse.

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