Pull out z-index to the sticker picker

As it's not a thing that's necessary for other PersistedElements,
only the sticker picker because it has to sit above the ContextMenu
it sits in.

src/components/views/elements/PersistedElement.js

@@ -34,9 +34,6 @@ function getOrCreateContainer(containerId) {
     return container;
 }
 
-// Greater than that of the ContextualMenu
-const PE_Z_INDEX = 5000;
-
 /*
  * Class of component that renders its children in a separate ReactDOM virtual tree
  * in a container element appended to document.body.
@@ -104,12 +101,11 @@ export default class PersistedElement extends React.Component {
             left: parentRect.left + 'px',
             width: parentRect.width + 'px',
             height: parentRect.height + 'px',
-            zIndex: PE_Z_INDEX,
         });
     }
 
     render() {
-        const content = <div ref={this.collectChild}>
+        const content = <div ref={this.collectChild} style={this.props.style}>
             {this.props.children}
         </div>;
 

src/components/views/rooms/Stickerpicker.js

@@ -27,6 +27,10 @@ import WidgetUtils from '../../../utils/WidgetUtils';
 
 const widgetType = 'm.stickerpicker';
 
+// We sit in a context menu, so the persisted element container needs to float
+// above it, so it needs a greater z-index than the ContextMenu
+const STICKERPICKER_Z_INDEX = 5000;
+
 export default class Stickerpicker extends React.Component {
     constructor(props) {
         super(props);
@@ -211,7 +215,7 @@ export default class Stickerpicker extends React.Component {
                             width: this.popoverWidth,
                         }}
                     >
-                    <PersistedElement persistKey="stickerPicker">
+                    <PersistedElement containerId="mx_persisted_stickerPicker" style={{zIndex: STICKERPICKER_Z_INDEX}}>
                         <AppTile
                             collectWidgetMessaging={this._collectWidgetMessaging}
                             id={stickerpickerWidget.id}