Only limit AutoComplete to window bounds if element is fixed.

src/components/layers/LayerSourceLayerBlock.jsx

@@ -12,17 +12,19 @@ class LayerSourceLayer extends React.Component {
     value: PropTypes.string,
     onChange: PropTypes.func,
     sourceLayerIds: PropTypes.array,
+    isFixed: PropTypes.bool,
   }
 
   static defaultProps = {
     onChange: () => {},
     sourceLayerIds: [],
+    isFixed: false
   }
 
   render() {
     return <InputBlock label={"Source Layer"} doc={styleSpec.latest.layer['source-layer'].doc}>
       <AutocompleteInput
-        keepMenuWithinWindowBounds={true}
+        keepMenuWithinWindowBounds={!!this.props.isFixed}
         value={this.props.value}
         onChange={this.props.onChange}
         options={this.props.sourceLayerIds.map(l => [l, l])}

src/components/modals/AddModal.jsx

@@ -127,6 +127,7 @@ class AddModal extends React.Component {
       }
       {this.state.type !== 'background' && this.state.type !== 'raster' &&
       <LayerSourceLayerBlock
+        isFixed={true}
         sourceLayerIds={layers}
         value={this.state['source-layer']}
         onChange={v => this.setState({ 'source-layer': v })}