[Text] TextCollapsingProperties corrections

.gitignore

@@ -225,3 +225,5 @@ src/Browser/Avalonia.Browser/staticwebassets
 # Claude agent worktrees
 .claude/worktrees/
 /.claude/settings.local.json
+/planning
+BenchmarkDotNet.Artifacts.*

src/Avalonia.Base/Media/GlyphRun.cs

@@ -684,15 +684,24 @@ private GlyphRunMetrics CreateGlyphRunMetrics()
             }
 
             var height = GlyphTypeface.Metrics.LineSpacing * Scale;
-            var widthIncludingTrailingWhitespace = 0d;
 
             var trailingWhitespaceLength = GetTrailingWhitespaceLength(isReversed, out var newLineLength, out var glyphCount);
 
-            for (var index = 0; index < _glyphInfos.Count; index++)
+            // A2: when our glyph source is a ShapedBuffer (the common case — every
+            // shape result flows through one), it already maintains a cluster-width
+            // prefix sum we can read in O(1) instead of summing all advances here.
+            double widthIncludingTrailingWhitespace;
+            if (_glyphInfos is TextFormatting.ShapedBuffer shapedBuffer)
             {
-                var advance = _glyphInfos[index].GlyphAdvance;
-
-                widthIncludingTrailingWhitespace += advance;
+                widthIncludingTrailingWhitespace = shapedBuffer.TotalGlyphAdvance;
+            }
+            else
+            {
+                widthIncludingTrailingWhitespace = 0d;
+                for (var index = 0; index < _glyphInfos.Count; index++)
+                {
+                    widthIncludingTrailingWhitespace += _glyphInfos[index].GlyphAdvance;
+                }
             }
 
             var width = widthIncludingTrailingWhitespace;

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

@@ -3,6 +3,21 @@
 
 namespace Avalonia.Media.TextFormatting;
 
+/// <summary>
+/// Walks the runs of a <see cref="TextLine"/> in <b>logical</b> (source-text)
+/// order. This is the order that splits and length-based offsets are defined
+/// in, and is what every <see cref="TextCollapsingProperties.Collapse"/>
+/// implementation needs to see — unlike <see cref="TextLine.TextRuns"/>,
+/// which exposes the post-BiDi <i>visual</i> ordering used for rendering.
+/// </summary>
+/// <remarks>
+/// When the line has been finalized (the normal case after
+/// <c>TextLineImpl.FinalizeLine</c>), the enumerator iterates over
+/// <c>_indexedTextRuns</c> — a level-resolved table that maps each run back
+/// to its original logical position. If the line hasn't been finalized (or
+/// the line is not a <c>TextLineImpl</c>), it falls back to the raw
+/// <see cref="TextLine.TextRuns"/> list.
+/// </remarks>
 internal ref struct LogicalTextRunEnumerator
 {
     private readonly IReadOnlyList<TextRun>? _textRuns;
@@ -61,7 +76,7 @@ public bool MoveNext([MaybeNullWhen(false)] out TextRun run)
         }
         else if (_textRuns != null)
         {
-            run = _textRuns[0];
+            run = _textRuns[_index];
         }
         else
         {