When the QWidget property editor is opened in Designer, the "Add X" buttons along the top look like this:

When you hover the mouse over the buttons, they have a hover effect, but stay that way until you click somewhere and hover over them again:

The expected behaviour is that they lose their hover effect when the mouse leaves them.

I tried to see if this was a Qt issue rather than it being the fault of some custom Creator button, but I was unable to recreate the buttons or find them in Creator's source.