Nearly every headless UI test opened with the same four-line prologue
(resolve the shared MainWindow, show it, cast its DataContext, wait for the
assembly list), then repeated the corelib lookup, the EnsureLazyChildren +
Children.OfType<T>().Single() drill, the registry single-by-header lookups,
and the open-an-assembly-and-wait dance. The duplication made the intent of
each test hard to see and every signature tweak a suite-wide edit.
Collapse those into TestHarness (BootAsync, OpenAssemblyAsync, GetCommand,
GetEntry) and TreeNavigation extensions (FindCoreLib, GetChild<T>, Expand<T>),
then apply them across the suite. Net ~865 lines of boilerplate removed with
no change in behaviour; the full headless suite stays green.
The MainMenu UserControl previously built a regular Avalonia Menu of
MenuItems, which on macOS would render inline in the window instead
of in the system menu bar -- not what Mac users expect. Avalonia's
NativeMenu + NativeMenuBar is the cross-platform abstraction: on
macOS the menu is projected into the system bar, on Windows / Linux
NativeMenuBar's presenter renders the same items inline. The MEF
registry, theme submenu, tool-pane toggles, and dynamic tab list all
flow through unchanged; only the leaf widget type swaps from MenuItem
to NativeMenuItem. KeyModifiers.Control is translated to Meta on
macOS so the system menu bar shows Cmd glyphs instead of Ctrl.
Assisted-by: Claude:claude-opus-4-7:Claude Code
Replaces the brittle root.GetVisualDescendants().OfType<T>().Single() / .First()
pattern with a new WaitForComponent<T>() extension that polls until the requested
control is in the visual tree, then returns it. Avalonia.Headless tests routinely
queried the visual tree before lazily-templated panes (DataGrid, dock content)
had materialised, surfacing as intermittent 'Sequence contains no elements'
failures across the suite.
Assisted-by: Claude:claude-opus-4-7:Claude Code
Siegfried Pammer