Found a workaround, was able to set the horizontal scrollbar to auto on feature attribute control.

ESRI.ArcGIS.Mobile.Client.Controls.ViewFeatureAttributesControl.ControlCreatingFeatureAttributes+=newEventHandler<ViewFeatureAttributesControlEventArgs>(ViewFeatureAttributesControl_ControlCreatingFeatureAttributes);voidViewFeatureAttributesControl_ControlCreatingFeatureAttributes(objectsender,ViewFeatureAttributesControlEventArgs e){((System.Windows.Controls.ScrollViewer)(((System.Windows.Controls.ContentControl)(e.Control)).Content)).水平面上crollBarVisibility=ScrollBarVisibility.Auto;}