useTwoPaneSemantics constant
If a RenderAbstractViewport overrides RenderObject.describeSemanticsConfiguration to add the SemanticsTag useTwoPaneSemantics to its SemanticsConfiguration, two semantics nodes will be used to represent the viewport with its associated scrolling actions in the semantics tree.
Two semantics nodes (an inner and an outer node) are necessary to exclude certain child nodes (via the excludeFromScrolling tag) from the scrollable area for semantic purposes: The SemanticsNodes of children that should be excluded from scrolling will be attached to the outer node. The semantic scrolling actions and the SemanticsNodes of scrollable children will be attached to the inner node, which itself is a child of the outer node.
Implementation
static const SemanticsTag useTwoPaneSemantics = SemanticsTag('RenderViewport.twoPane')