extend
Marks this window to have its content extend vertically into the title pane area. Use the following methods for finer control over such extended content:
Window.getTitlePaneControlInsets to query the insets that should be reserved for the main control buttons - close / maximize / minimize.
Window.setPreferredTitlePaneHeight to increase the preferred height of the title pane area in case the content you extend into that area is taller than the main control buttons.
Window.createTitlePaneControlButton to get a button that has consistent visual appearance and preferred size with the main control buttons.
Parameters
Horizontal gravity for the title control buttons. Must be either RadianceThemingSlices.HorizontalGravity.LEADING or RadianceThemingSlices.HorizontalGravity.TRAILING.
Vertical gravity for the title control buttons. May not be null.