It may start working if you move your mouse over the button a few times. If it doesn't, open the UI from the extension popup.
Ultrawidify
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Change when and if this popup appearsThis menu option will show {{settings.active.newFeatureTracker?.['uw6.ui-popup']?.show}} more times; or until clicked or dismissed.
Also accessible via: EXTENSION SETTINGS > UI AND KEYBOARD.
Dismiss this option