fix: last line change may disable hunk operation in side-by-side diff (#2027)

Signed-off-by: leo <[email protected]>

Author: love-linger

src/ViewModels/TextDiffContext.cs

@@ -223,7 +223,7 @@ public override TextDiffContext SwitchMode()
             return new CombinedTextDiff(_option, _data, this);
         }
 
-        public void ConvertsToCombinedRange(ref int startLine, ref int endLine, bool isOldSide)
+        public void GetCombinedRangeForSingleSide(ref int startLine, ref int endLine, bool isOldSide)
         {
             endLine = Math.Min(endLine, _data.Lines.Count - 1);
 
@@ -262,6 +262,32 @@ public void ConvertsToCombinedRange(ref int startLine, ref int endLine, bool isO
             endLine = _data.Lines.IndexOf(endContent);
         }
 
+        public void GetCombinedRangeForBothSides(ref int startLine, ref int endLine, bool isOldSide)
+        {
+            var fromSide = isOldSide ? Old : New;
+            endLine = Math.Min(endLine, fromSide.Count - 1);
+
+            // Since this function is only used for auto-detected hunk, we just need to find out the a first changed line
+            // and then use `FindRangeByIndex` to get the range of hunk.
+            for (int i = startLine; i <= endLine; i++)
+            {
+                var line = fromSide[i];
+                if (line.Type == Models.TextDiffLineType.Added || line.Type == Models.TextDiffLineType.Deleted)
+                {
+                    (startLine, endLine) = FindRangeByIndex(_data.Lines, _data.Lines.IndexOf(line));
+                    return;
+                }
+
+                if (line.Type == Models.TextDiffLineType.None)
+                {
+                    var otherSide = isOldSide ? New : Old;
+                    var changedLine = otherSide[i]; // Find the changed line on the other side in the same position
+                    (startLine, endLine) = FindRangeByIndex(_data.Lines, _data.Lines.IndexOf(changedLine));
+                    return;
+                }
+            }
+        }
+
         private void FillEmptyLines()
         {
             if (Old.Count < New.Count)

src/Views/TextDiffView.axaml.cs

@@ -1183,7 +1183,7 @@ protected override void UpdateSelectedChunk(double y)
                     endLine.GetTextLineVisualYPosition(endLine.TextLines[^1], VisualYPosition.TextBottom) - view.VerticalOffset :
                     view.Bounds.Height;
 
-                diff.ConvertsToCombinedRange(ref startIdx, ref endIdx, IsOld);
+                diff.GetCombinedRangeForSingleSide(ref startIdx, ref endIdx, IsOld);
                 TrySetChunk(new(rectStartY, rectEndY - rectStartY, startIdx, endIdx, false, IsOld));
             }
             else
@@ -1229,7 +1229,7 @@ protected override void UpdateSelectedChunk(double y)
                     endLine.GetTextLineVisualYPosition(endLine.TextLines[^1], VisualYPosition.TextBottom) - view.VerticalOffset :
                     view.Bounds.Height;
 
-                diff.ConvertsToCombinedRange(ref startIdx, ref endIdx, IsOld);
+                diff.GetCombinedRangeForBothSides(ref startIdx, ref endIdx, IsOld);
                 TrySetChunk(new(rectStartY, rectEndY - rectStartY, startIdx, endIdx, true, false));
             }
         }