gtk:tool-button-use-underline (object)

object -- a gtk:tool-button widget
setting -- a boolean whether the button label has the form "_Open"

The accessor for the use-underline slot gets or sets whether underscores in the label property are used as mnemonics on menu items on the overflow menu. If set, an underline in the label property indicates that the next character should be used for the mnemonic accelerator key in the overflow menu.

Labels shown on tool buttons never have mnemonics on them. This property only affects the menu item on the overflow menu.