Fix broken pre-edit markedText on iOS and solve #17523 (#17618)

* fix: add missing  ref keyword to CombinedSpan3.CopyFromSpan

* fix: fix incorrect indent in IUITextInput.TextInRange and boundary condition for surroundingText

---------

Co-authored-by: Max Katz <maxkatz6@outlook.com>

src/iOS/Avalonia.iOS/CombinedSpan3.cs

@@ -16,7 +16,7 @@ internal ref struct CombinedSpan3<T>
 
     public int Length => Span1.Length + Span2.Length + Span3.Length;
 
-    static void CopyFromSpan(ReadOnlySpan<T> from, int offset, ref Span<T> to)
+    static void CopyFromSpan(ReadOnlySpan<T> from, ref int offset, ref Span<T> to)
     {
         if(to.Length == 0)
             return;
@@ -33,8 +33,8 @@ internal ref struct CombinedSpan3<T>
     
     public void CopyTo(Span<T> to, int offset)
     {
-        CopyFromSpan(Span1, offset, ref to);
-        CopyFromSpan(Span2, offset, ref to);
-        CopyFromSpan(Span3, offset, ref to);
+        CopyFromSpan(Span1, ref offset, ref to);
+        CopyFromSpan(Span2, ref offset, ref to);
+        CopyFromSpan(Span3, ref offset, ref to);
     }
 }

src/iOS/Avalonia.iOS/TextInputResponder.cs

@@ -220,15 +220,17 @@ partial class AvaloniaView
 
             string result = "";
             if (string.IsNullOrEmpty(_markedText))
+            {
                 if(surroundingText != null && r.EndIndex < surroundingText.Length)
                 {
                     result = surroundingText[r.StartIndex..r.EndIndex];
                 }
+            }
             else
             {
                 var span = new CombinedSpan3<char>(surroundingText.AsSpan().Slice(0, currentSelection.Start),
                     _markedText,
-                    surroundingText.AsSpan().Slice(currentSelection.Start));
+                    surroundingText.AsSpan().Slice(currentSelection.Start, currentSelection.End - currentSelection.Start));
                 var buf = new char[r.EndIndex - r.StartIndex];
                 span.CopyTo(buf, r.StartIndex);
                 result = new string(buf);