Fix StackOverflow when a NaN offset is set on ScrollViewer (#21558)

ScrollViewer.Offset and ScrollContentPresenter.Offset form a two-way coerced
binding whose convergence is decided with the default equality comparer. Because
NaN != NaN, a NaN offset is reported "changed" on every pass, so the coerce/raise
cycle never converges and recurses until the stack overflows -- an unrecoverable
crash (on Mono/AOT targets such as Android it surfaces as a native SIGSEGV).

CoerceOffset's Clamp passed NaN straight through (NaN < min and NaN > max are
both false), so a NaN component survived coercion. Sanitize it to the lower
bound so the coerce always converges, regardless of where the NaN came from.

Fixes #21444

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

src/Avalonia.Controls/ScrollViewer.cs

@@ -696,6 +696,14 @@ namespace Avalonia.Controls
 
         private static double Clamp(double value, double min, double max)
         {
+            // A NaN offset must never survive. Offset is two-way coerced between the ScrollViewer and its
+            // ScrollContentPresenter; because NaN != NaN, a NaN offset never compares equal, so the
+            // coerce/raise cycle never converges and recurses until it overflows the stack (see #21444).
+            if (double.IsNaN(value))
+            {
+                return min;
+            }
+
             return (value < min) ? min : (value > max) ? max : value;
         }
 

tests/Avalonia.Controls.UnitTests/ScrollViewerTests.cs

@@ -44,6 +44,25 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Vector(10, 10), target.Offset);
         }
 
+        [Fact]
+        public void Setting_Offset_To_NaN_Does_Not_Cause_Infinite_Coerce_Recursion()
+        {
+            var target = new ScrollViewer
+            {
+                Template = new FuncControlTemplate<ScrollViewer>(CreateTemplate),
+                Content = "Foo",
+                Extent = new Size(100, 100),
+                Viewport = new Size(10, 10),
+            };
+
+            InitializeScrollViewer(target);
+
+            target.Offset = new Vector(0, double.NaN);
+
+            Assert.False(double.IsNaN(target.Offset.X));
+            Assert.False(double.IsNaN(target.Offset.Y));
+        }
+
         [Fact]
         public void Test_ScrollToHome()
         {