Add ScrollableGroups

Author: krice-nw

reference/scenegraph.md

@@ -63,6 +63,7 @@ function myCommand(selection) {
     * [Group](#Group)
     * [SymbolInstance](#SymbolInstance)
     * [RepeatGrid](#RepeatGrid)
+    * [ScrollableGroup](#ScrollableGroup)
     * [LinkedGraphic](#LinkedGraphic)
     * [RootNode](#RootNode)
 
@@ -2195,6 +2196,56 @@ You can call this API from either of _two different edit contexts_:
 
 * * *
 
+<a name="ScrollableGroup"></a>
+
+## ScrollableGroup
+**Since:** XD 30
+**Kind**: class
+**Extends**: [<code>SceneNode</code>](#SceneNode)
+
+ScrollableGroup nodes are content that users can interactively scroll around. Content is viewed through a {@link viewport},
+with everything else clipped. If a ScrollableGroup is set to only scroll on one axis, on the other axis the viewport is
+automatically sized to exactly fit the bounds of the content so nothing is clipped.
+
+The scroll distance range is defined by a _scrollable area_ rectangle which is the union of the viewport and the bounds of all
+the content. This can include some blank space, if the content is initially positioned not filling the entire viewport.
+
+* [ScrollableGroup](#ScrollableGroup)
+    * [.scrollingType](#ScrollableGroup-scrollingType) : <code>string</code>
+    * [.viewport](#ScrollableGroup-viewport) : <code>{! {viewportWidth: number, offsetX: number} | {viewportHeight: number, offsetY: number} |
+         {viewportWidth: number, offsetX: number, viewportHeight: number, offsetY: number} }</code>
+
+* * *
+
+<a name="ScrollableGroup-scrollingType"></a>
+
+### repeatGrid.scrollingType : <code>string</code>
+The type of scrolling: one of ScrollableGroup.VERTICAL, HORIZONTAL and PANNING.
+        PANNING enables scrolling on both axes.
+
+**Kind**: instance property of [<code>ScrollableGroup</code>](#ScrollableGroup)
+
+* * *
+
+<a name="ScrollableGroup-viewport"></a>
+
+### ScrollableGroup.viewport : <code>string</code>
+The viewport is a rectangle whose bounds are defined explicitly on scrolling axes and fit automatically to the
+content on non-scrolling axes:
+* On a scrolling axis, the bounds are specified in {@link core/coordinate-spaces-and-units.md local coordinates}
+using the `viewport` values specified here.
+* On a non-scrolling axis, the bounds are automatically calculated to exactly fit the content (just like the blue
+selection rectangle seen when you select a plain Group).
+
+For example, if scrollingType == VERTICAL, the top of the viewport is `viewport.offsetY` in the ScrollableGroup's
+local coordinates, the bottom of the viewport is `viewport.offsetY + viewport.viewportHeight` in local coordinates,
+and horizontally there is no viewport clipping -- the entire current {@link localBounds} range is visible. The
+`viewport` object will only contain `offsetY` and `viewportHeight` properties in this case.
+
+**Kind**: instance property of [<code>ScrollableGroup</code>](#ScrollableGroup)
+
+* * *
+
 <a name="LinkedGraphic"></a>
 
 ## LinkedGraphic