Make sure a GlyphRun's bounds are properly calculated

src/Avalonia.Visuals/Media/GlyphRun.cs

@@ -18,7 +18,7 @@ namespace Avalonia.Media
         private double _fontRenderingEmSize;
         private Size? _size;
         private int _biDiLevel;
-        private Point _baselineOrigin;
+        private Point? _baselineOrigin;
 
         private ReadOnlySlice<ushort> _glyphIndices;
         private ReadOnlySlice<double> _glyphAdvances;
@@ -97,7 +97,9 @@ namespace Avalonia.Media
         {
             get
             {
-                return _baselineOrigin;
+                _baselineOrigin ??= CalculateBaselineOrigin();
+
+                return _baselineOrigin.Value;
             }
             set => Set(ref _baselineOrigin, value);
         }
@@ -538,6 +540,15 @@ namespace Avalonia.Media
             return GlyphAdvances[index];
         }
 
+        /// <summary>
+        /// Calculates the default baseline origin of the <see cref="GlyphRun"/>.
+        /// </summary>
+        /// <returns>The baseline origin.</returns>
+        private Point CalculateBaselineOrigin()
+        {
+            return new Point(0, -GlyphTypeface.Ascent * Scale);
+        }
+
         /// <summary>
         /// Calculates the size of the <see cref="GlyphRun"/>.
         /// </summary>

src/Avalonia.Visuals/Media/TextFormatting/TextLineImpl.cs

@@ -39,7 +39,7 @@ namespace Avalonia.Media.TextFormatting
 
             foreach (var textRun in _textRuns)
             {
-                var offsetY = LineMetrics.TextBaseline;
+                var offsetY = LineMetrics.TextBaseline - textRun.GlyphRun.BaselineOrigin.Y;
 
                 using (drawingContext.PushPostTransform(Matrix.CreateTranslation(currentX, offsetY)))
                 {