PopupTool: Fix popup z-index override
PopupTool: Fix popup z-index override

Follow-up to f11c7faa2a1034e4ab455047e59e1ed1a82a9f97.

Bug: T222427
Change-Id: Ic90fdc3fb3f5aeb74b937abf2e42999606bf9cd5

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui