combobox: Add a special class for the button in the combobox
That way, the GTK engine doesn't think that the general .button CSS might potentially apply to it. And because combobox button is overly complex and stupid, it cannot be cached. So buttons thought they cannot ever cache anything because they might suddenly end up inside a combobox without noticing and then they'd need to round their corners differently. Of course they're just regular "Remove" buttons like all the other 100s of "Remove" buttons in gnome-software. But hey, better not cache anything for them and recompute their CSS every time the :hover state changes on one of the rows.
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