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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.

More info on splitters in the XUL tutorial