If the modifier is pressed and release without another event, the
intended behaviour is to clear an urgency hint and hide the bar again.
Note that if multiple workspaces have the urgency hint set, the bar is
hidden again and an urgent workspace is focused, the bar does not stay
hidden anymore.
Add a second reason to show the bar besides visible by modifier.
Update the visibility based on changes in the workspace urgency.
Check all workspaces for urgency and keep the bar visible if at least
one has an urgency hint.
