Base class for [AnimationNode]s with multiple input ports that must be synchronized. An animation node used to combine, mix, or blend two or more animations together while keeping them synchronized within an [AnimationTree]. $DOCS_URL/tutorials/animation/animation_tree.html If [code]false[/code], the blended animations' frame are stopped when the blend value is [code]0[/code]. If [code]true[/code], forcing the blended animations to advance frame.