Fix caption hit test.

src/Windows/Avalonia.Win32/WindowImpl.CustomCaptionProc.cs

@@ -44,12 +44,25 @@ namespace Avalonia.Win32
             ushort uCol = 1;
             bool onResizeBorder = false;
 
+            // Determine if the point is at the left or right of the window.
+            if (ptMouse.X >= rcWindow.left && ptMouse.X < rcWindow.left + borderThickness.left)
+            {
+                uCol = 0; // left side
+            }
+            else if (ptMouse.X < rcWindow.right && ptMouse.X >= rcWindow.right - borderThickness.right)
+            {
+                uCol = 2; // right side
+            }
+
             // Determine if the point is at the top or bottom of the window.
             if (ptMouse.Y >= rcWindow.top && ptMouse.Y < rcWindow.top + borderThickness.top)
             {
                 onResizeBorder = (ptMouse.Y < (rcWindow.top - rcFrame.top));
 
-                if (onResizeBorder)
+                // Two cases where we have a valid row 0 hit test:
+                // - window resize border (top resize border hit)
+                // - area below resize border that is actual titlebar (caption hit).
+                if (onResizeBorder || uCol == 1)
                 {
                     uRow = 0;
                 }
@@ -59,16 +72,6 @@ namespace Avalonia.Win32
                 uRow = 2;
             }
 
-            // Determine if the point is at the left or right of the window.
-            if (ptMouse.X >= rcWindow.left && ptMouse.X < rcWindow.left + borderThickness.left)
-            {
-                uCol = 0; // left side
-            }
-            else if (ptMouse.X < rcWindow.right && ptMouse.X >= rcWindow.right - borderThickness.right)
-            {
-                uCol = 2; // right side
-            }
-
             ReadOnlySpan<HitTestValues> hitZones = stackalloc HitTestValues[]
             {
                 HitTestValues.HTTOPLEFT, onResizeBorder ? HitTestValues.HTTOP : HitTestValues.HTCAPTION,