test(launcher): Campaign LA — headless MainWindow view tests close #399

#398 was a crash on every modal open/close caused by MainWindow's
constructor calling AvaloniaXamlLoader.Load(this) instead of the
generated InitializeComponent() — only InitializeComponent assigns the
x:Name backing fields, so every named control was null and the first
Dispatcher.UIThread.Post callback in OnViewModelPropertyChanged threw
NullReferenceException, killing the process. It reached the user gate
because no test in tests/AcDream.Launcher.Tests (ViewModel-only) ever
constructed a MainWindow. #399 is the process gap that let that class of
defect through 14,012 green tests.

Adds Avalonia.Headless.XUnit 12.1.1 to the launcher test project. Its
net10.0 dependency group targets xunit v3, so the project migrates
xunit 2.9.3 -> xunit.v3 3.2.2 (drop-in: all 54 pre-existing tests compile
and pass unchanged under dotnet test via xunit.runner.visualstudio 3.1.4,
which already supported v1/v2/v3; two call sites needed
TestContext.Current.CancellationToken per the new xUnit1051 analyzer).
TestAppBuilder.cs wires [assembly: AvaloniaTestApplication] to a headless
AppBuilder.Configure<App>() so the real App.axaml FluentTheme is live in
tests.

MainWindowViewTests.cs adds 12 [AvaloniaFact]/[AvaloniaTheory] tests:
- an explicit non-null + type check of every x:Name field the
  code-behind dereferences (ProfilesTree, ServerNameTextBox,
  AccountNameTextBox, CharacterNameTextBox, EditorSubmitButton,
  FirstRunDatDirectoryTextBox, FirstRunCloseButton, UpdateCloseButton)
- a reflection sweep over every x:Name found in MainWindow.axaml, so a
  future named control without a matching non-null field fails loudly
- one open+close round trip per ProfileEditorKind (all seven, including
  Remove), plus the first-run wizard and the update prompt, each pumping
  Dispatcher.UIThread.RunJobs() so the queued focus callback actually
  executes instead of just being asserted vacuously
- a dedicated test for the _focusBeforeModal-restore branch (not just
  the ProfilesTree.Focus() fallback), anchored on a real focusable
  button since ProfilesTree (TreeView) has Focusable="False" under
  FluentTheme — its own tab stops are TreeViewItem rows, so the
  close-path assertions check "no exception escaped the dispatcher"
  rather than "focus landed on ProfilesTree"

Falsification (required evidence): reverting MainWindow's constructor to
AvaloniaXamlLoader.Load(this) and rerunning gives 12 failed / 0 passed —
10 tests throw NullReferenceException at MainWindow.FocusActiveModal,
propagating cleanly out of Dispatcher.UIThread.RunJobs() (confirming
dispatcher exceptions are not silently swallowed), and the 2 reflection
tests fail on an explicit "x:Name 'ProfilesTree' was null after
construction" message. Restoring InitializeComponent() gives 12 passed /
0 failed. Full launcher suite: 66 passed / 0 failed, reproduced on both
Windows and native Ubuntu (WSL, no display/Xvfb — Avalonia.Headless needs
none). AcDream.Launcher.Core.Tests: 317/317 unaffected.

No CI workflow change needed: .github/workflows/headless-portability.yml's
portable-launcher job already runs dotnet test on the launcher test
project on both windows-latest and ubuntu-latest with no display setup,
which is sufficient for Avalonia.Headless.

Closes #399.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ISSUES.md

@@ -26,7 +26,8 @@ What does NOT go here:
 
 ## #399 — Launcher: no test ever constructs MainWindow, so code-behind defects reach the user gate
 
-**Status:** OPEN
+**Status:** DONE (this commit, Campaign LA UI-test slice) — closed via
+`tests/AcDream.Launcher.Tests/MainWindowViewTests.cs`.
 **Severity:** HIGH (process class: this gap let #398 — a crash on every
 modal open/close — pass 14,012 green tests and reach the user gate)
 **Filed:** 2026-08-15 (found while launching the launcher for the LA11 gate)
@@ -38,14 +39,50 @@ view. `LauncherWindowViewModelTests` proved the modal state machine while
 the code-behind that consumes it was never executed once, which is exactly
 how #398's null `x:Name` fields survived every automated gate.
 
-**Fix direction.** Add `Avalonia.Headless.XUnit` to the launcher test
-project and a focused view test that constructs `MainWindow`, asserts every
-`x:Name` field the code-behind dereferences is non-null, and drives each
-modal (editor kinds, first-run wizard, update prompt) through open and
-close so the focus/restore paths actually run. That catches the class —
-any null named control or throwing handler — not just #398's spelling.
+**Fix landed.** Added `Avalonia.Headless.XUnit` 12.1.1 to the launcher test
+project (its net10.0 dependency group targets **xunit v3**, so the project
+migrated `xunit` 2.9.3 → `xunit.v3` 3.2.2 — a drop-in swap; all 54
+pre-existing `[Fact]`/`[Theory]`/`Assert.*` tests compiled and passed
+unchanged, only two call sites needed `TestContext.Current.CancellationToken`
+per the new `xUnit1051` analyzer). `TestAppBuilder`
+(`tests/AcDream.Launcher.Tests/TestAppBuilder.cs`) wires
+`[assembly: AvaloniaTestApplication]` to a headless `AppBuilder.Configure<App>()`
+so FluentTheme (declared in the real `App.axaml`) is live for every test.
+`MainWindowViewTests.cs` adds 12 `[AvaloniaFact]`/`[AvaloniaTheory]` tests:
+an explicit non-null check of every `x:Name` field the code-behind
+dereferences, a reflection sweep over every `x:Name` in the markup (so a
+future named control without a matching non-null field fails loudly), and
+one open+close round trip per `ProfileEditorKind` (all seven, including
+`Remove`) plus the first-run wizard and the update prompt — each pumping
+`Dispatcher.UIThread.RunJobs()` so the `Dispatcher.UIThread.Post` callback
+in `OnViewModelPropertyChanged`/`FocusActiveModal` actually executes, not
+just gets queued. A dedicated test proves the `_focusBeforeModal != null`
+restore branch (not just the `ProfilesTree.Focus()` fallback) also runs
+clean, anchored on a real focusable button since `ProfilesTree` (a
+`TreeView`) has `Focusable="False"` under FluentTheme — its own tab stops
+are `TreeViewItem` rows, so the close-path assertions check "no exception
+escaped" rather than "focus landed on ProfilesTree" (that would be a false
+expectation, not the bug this issue is about).
 
-**Acceptance:** a headless view test fails against the pre-#398 code
+**Falsification (required evidence).** Reverting `MainWindow`'s constructor
+to `AvaloniaXamlLoader.Load(this)` and rerunning: **12 failed / 0 passed**
+— 10 tests throw `System.NullReferenceException` at
+`AcDream.Launcher.MainWindow.FocusActiveModal` (propagating cleanly out of
+`Dispatcher.UIThread.RunJobs()`, confirming dispatcher exceptions are not
+swallowed), the 2 reflection tests fail on an explicit
+"`x:Name 'ProfilesTree' was null after construction`" message. Restoring
+`InitializeComponent()`: **12 passed / 0 failed**. Full launcher suite:
+**66 passed / 0 failed** (Windows and native Ubuntu/WSL, both post-fix).
+`tests/AcDream.Launcher.Core.Tests`: 317/317 unaffected.
+
+**CI.** `.github/workflows/headless-portability.yml`'s `portable-launcher`
+job already runs `dotnet test tests/AcDream.Launcher.Tests/...` on both
+`windows-latest` and `ubuntu-latest` with no display setup — no workflow
+change was needed, since `Avalonia.Headless` requires no real windowing
+system (confirmed directly: the new tests pass unmodified under WSL/native
+Linux with no `DISPLAY` or Xvfb).
+
+**Acceptance (met):** a headless view test fails against the pre-#398 code
 (`AvaloniaXamlLoader.Load`) and passes after, and runs in the portable
 Windows+Ubuntu CI lane alongside the existing launcher tests.