Fixes #4125 - Decouple ApplicationImpl from static Application references (#4324)

* Initial plan

* Add instance fields to ApplicationImpl and update static Application properties to delegate to ApplicationImpl.Instance

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Fix code review issues: use instance fields in Run() and improve Shutdown() logic

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Final code review fix: use _initialized directly in Shutdown

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Fix Shutdown order: call ResetState before clearing instance fields

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Add MainThreadId as instance field in ApplicationImpl

Co-authored-by: tig <585482+tig@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tig <585482+tig@users.noreply.github.com>
Co-authored-by: Tig <tig@users.noreply.github.com>

Tests/UnitTests/Application/ApplicationImplTests.cs

@@ -583,4 +583,52 @@ public class ApplicationImplTests
 
         Assert.True (result);
     }
+
+    [Fact]
+    public void ApplicationImpl_UsesInstanceFields_NotStaticReferences()
+    {
+        // This test verifies that ApplicationImpl uses instance fields instead of static Application references
+        var orig = ApplicationImpl.Instance;
+
+        var v2 = NewApplicationImpl();
+        ApplicationImpl.ChangeInstance(v2);
+
+        // Before Init, all fields should be null/default
+        Assert.Null(v2.Driver);
+        Assert.False(v2.Initialized);
+        Assert.Null(v2.Popover);
+        Assert.Null(v2.Navigation);
+        Assert.Null(v2.Top);
+        Assert.Empty(v2.TopLevels);
+
+        // Init should populate instance fields
+        v2.Init();
+
+        // After Init, Driver, Navigation, and Popover should be populated
+        Assert.NotNull(v2.Driver);
+        Assert.True(v2.Initialized);
+        Assert.NotNull(v2.Popover);
+        Assert.NotNull(v2.Navigation);
+        Assert.Null(v2.Top); // Top is still null until Run
+
+        // Verify that static Application properties delegate to instance
+        Assert.Equal(v2.Driver, Application.Driver);
+        Assert.Equal(v2.Initialized, Application.Initialized);
+        Assert.Equal(v2.Popover, Application.Popover);
+        Assert.Equal(v2.Navigation, Application.Navigation);
+        Assert.Equal(v2.Top, Application.Top);
+        Assert.Same(v2.TopLevels, Application.TopLevels);
+
+        // Shutdown should clean up instance fields
+        v2.Shutdown();
+
+        Assert.Null(v2.Driver);
+        Assert.False(v2.Initialized);
+        Assert.Null(v2.Popover);
+        Assert.Null(v2.Navigation);
+        Assert.Null(v2.Top);
+        Assert.Empty(v2.TopLevels);
+
+        ApplicationImpl.ChangeInstance(orig);
+    }
 }