Fix fallback handling

src/Avalonia.Base/Media/TextFormatting/TextCharacters.cs

@@ -74,6 +74,15 @@ namespace Avalonia.Media.TextFormatting
 
             if (TryGetShapeableLength(text, currentTypeface, null, out var count, out var script))
             {
+                if (script == Script.Common && previousTypeface is not null)
+                {
+                    if (TryGetShapeableLength(text, previousTypeface.Value, null, out var fallbackCount, out _))
+                    {
+                        return new ShapeableTextCharacters(text.Take(fallbackCount),
+                            defaultProperties.WithTypeface(previousTypeface.Value), biDiLevel);
+                    }
+                }
+
                 return new ShapeableTextCharacters(text.Take(count), defaultProperties.WithTypeface(currentTypeface),
                     biDiLevel);
             }
@@ -173,7 +182,7 @@ namespace Avalonia.Media.TextFormatting
 
                 var currentScript = currentGrapheme.FirstCodepoint.Script;
 
-                if (defaultFont != null && defaultFont.TryGetGlyph(currentGrapheme.FirstCodepoint, out _))
+                if (!currentGrapheme.FirstCodepoint.IsWhiteSpace && defaultFont != null && defaultFont.TryGetGlyph(currentGrapheme.FirstCodepoint, out _))
                 {
                     break;
                 }

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -332,8 +332,6 @@ namespace Avalonia.Controls.Presenters
 
         protected override bool BypassFlowDirectionPolicies => true;
 
-        public ComposingRegion? ComposingRegion { get; internal set; }
-
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
         /// </summary>
@@ -553,7 +551,7 @@ namespace Avalonia.Controls.Presenters
 
             if (!string.IsNullOrEmpty(_preeditText))
             {
-                var preeditHighlight = new ValueSpan<TextRunProperties>(ComposingRegion.HasValue ? ComposingRegion.Value.Start : _caretIndex, _preeditText.Length,
+                var preeditHighlight = new ValueSpan<TextRunProperties>(_caretIndex, _preeditText.Length,
                         new GenericTextRunProperties(typeface, FontSize,
                         foregroundBrush: foreground,
                         textDecorations: TextDecorations.Underline));
@@ -880,7 +878,7 @@ namespace Avalonia.Controls.Presenters
             }
             else
             {
-                var textPosition = (ComposingRegion.HasValue? ComposingRegion.Value.Start : _caretIndex) + newValue?.Length ?? 0;
+                var textPosition = _caretIndex + newValue?.Length ?? 0;
 
                 var characterHit = GetCharacterHitFromTextPosition(textPosition);
 

src/Avalonia.Controls/TextBoxTextInputMethodClient.cs

@@ -108,11 +108,6 @@ namespace Avalonia.Controls
             }
 
             _presenter.PreeditText = text;
-
-            if(text == null)
-            {
-                _presenter.ComposingRegion = null;
-            }
         }
 
         public void SelectInSurroundingText(int start, int end)