<Panel MinWidth="400" MinHeight="260">
  <Menu VerticalAlignment="Top">
    <MenuItem Header="File">
      <MenuItem Header="New File" InputGesture="Ctrl+N" />
      <MenuItem Header="Open Recent">
        <MenuItem Header="Avalonia UIKit" />
        <MenuItem Header="Sample App" />
      </MenuItem>
      <Separator />
      <MenuItem Header="Save" InputGesture="Ctrl+S" />
      <MenuItem Header="Save As…" IsEnabled="False" />
      <Separator />
      <MenuItem Header="Quit" />
    </MenuItem>
    <MenuItem Header="Edit">
      <MenuItem Header="Undo" InputGesture="Ctrl+Z" />
      <MenuItem Header="Redo" InputGesture="Ctrl+Shift+Z" />
      <Separator />
      <MenuItem Header="Word Wrap" ToggleType="CheckBox" IsChecked="True" />
    </MenuItem>
    <MenuItem Header="View">
      <MenuItem Header="Zoom In" />
      <MenuItem Header="Zoom Out" />
    </MenuItem>
  </Menu>
</Panel>

Usage

The theme covers Menu, MenuItem, ContextMenu and MenuFlyoutPresenter in full: the popover surface with its hairline ring and shadow, 26px items 2px apart, an icon column when any item has an Icon or a check, ToggleType="CheckBox" and "Radio" items, submenus with a chevron, Separators, and InputGesture shown as a shortcut. Menu is a menu bar drawn in the window: its top-level items are small ghost buttons, shown selected while open.

Two settings are left to the app. Submenus open as soon as they are hovered: set DefaultMenuInteractionHandler.MenuShowDelay to zero at startup, as Avalonia waits otherwise:

Avalonia.Controls.Platform.DefaultMenuInteractionHandler.MenuShowDelay = TimeSpan.Zero;

And open a MenuFlyout with Placement="BottomEdgeAlignedLeft" (BottomEdgeAlignedRight for a split button): the presenter keeps a 4px gap from the trigger. A ContextMenu opens at the pointer. Avalonia selects the first item when a menu opens or takes focus; the theme gives that selection the hover look.