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])}