[Backport staging] Fix Stickerpicker layout crossing multiple CSS stacking contexts (#12127)

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/views/rooms/Stickerpicker.tsx

@@ -356,6 +356,7 @@ export default class Stickerpicker extends React.PureComponent<IProps, IState> {
                 menuPaddingLeft={0}
                 menuPaddingRight={0}
                 zIndex={STICKERPICKER_Z_INDEX}
+                mountAsChild={true}
                 {...this.props.menuPosition}
             >
                 <GenericElementContextMenu element={this.getStickerpickerContent()} onResize={this.onFinished} />