fix: Expose offset property on JS TreeViewportData (deephaven#6639)

Fixes DH-18523

Author: niloc132

web/client-api/src/main/java/io/deephaven/web/client/api/subscription/AbstractTableSubscription.java

@@ -411,9 +411,9 @@ public UpdateEventData(WebBarrageSubscription subscription, int rowStyleColumn,
                     subscription.getServerViewport(), subscription.isReversed()));
         }
 
-        // for ViewportData
+        // for ViewportData, TreeViewportData
         @JsProperty
-        public Double getOffset() {
+        public double getOffset() {
             return offset;
         }
 

web/client-api/src/main/java/io/deephaven/web/client/api/subscription/ViewportData.java

@@ -22,10 +22,10 @@
 public interface ViewportData extends TableData {
 
     /**
-     * The position of the first returned row, null if this data is not for a viewport.
+     * The position of the first returned row within the table.
      */
     @JsProperty
-    Double getOffset();
+    double getOffset();
 
     @JsProperty
     @Override

web/client-api/src/main/java/io/deephaven/web/client/api/tree/TreeViewportData.java

@@ -9,6 +9,10 @@
 import jsinterop.annotations.JsProperty;
 import jsinterop.annotations.JsType;
 
+/**
+ * Similar to {@link io.deephaven.web.client.api.subscription.ViewportData}, but with additional properties to reflect
+ * the tree structure.
+ */
 @JsType(namespace = "dh")
 public interface TreeViewportData extends TableData {
     @JsProperty
@@ -24,6 +28,12 @@ default TableData.Row get(RowPositionUnion index) {
     @Override
     JsArray<TableData.@TsTypeRef(TreeRow.class) Row> getRows();
 
+    /**
+     * The position of the first returned row within the tree.
+     */
+    @JsProperty
+    double getOffset();
+
     /**
      * Row implementation that also provides additional read-only properties. represents visible rows in the table, but
      * with additional properties to reflect the tree structure.