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As of writing the button with label _Add is displayed as _Add, this seems to be an issue with the SDK's CI not having current libadwaita main branch in which use-underline is set to True by default on the button.
As of writing the button with label _Add is displayed as _Add, this seems to be an issue with the SDK's CI not having current libadwaita main branch in which use-underline is set to True by default on the button.