Add navigation and highlighting of change-blocks in Text Diff (#616, #717) (#703)

* Corrected misspelled local variable nextHigh(t)light

* Implemented change-block navigation

* Modified behavior of the Prev/Next Change buttons in DiffView toolbar.
* Well-defined change-blocks are pre-calculated and can be navigated between.
* Current change-block is highlighted in the Diff panel(s).
* Prev/next at start/end of range (re-)scrolls to first/last change-block
(I.e when unset, or already at first/last change-block, or at the only one.)
* Current change-block is unset in RefreshContent().

* Added safeguards for edge cases

* Added indicator of current/total change-blocks in DiffView toolbar

* Added new Icon and String (en-US) for Highlighted Diff Navigation

* Added Preference and ToggleButton for diff navigation style

src/Models/DiffResult.cs

@@ -2,6 +2,8 @@
 using System.Text;
 using System.Text.RegularExpressions;
 
+using CommunityToolkit.Mvvm.ComponentModel;
+
 using Avalonia;
 using Avalonia.Media.Imaging;
 
@@ -59,16 +61,70 @@ namespace SourceGit.Models
         }
     }
 
-    public partial class TextDiff
+    public class TextDiffChangeBlock
+    {
+        public TextDiffChangeBlock(int startLine, int endLine)
+        {
+            StartLine = startLine;
+            EndLine = endLine;
+        }
+
+        public int StartLine { get; set; } = 0;
+        public int EndLine { get; set; } = 0;
+
+        public bool IsInRange(int line)
+        {
+            return line >= StartLine && line <= EndLine;
+        }
+    }
+
+    public partial class TextDiff : ObservableObject
     {
         public string File { get; set; } = string.Empty;
         public List<TextDiffLine> Lines { get; set; } = new List<TextDiffLine>();
         public Vector ScrollOffset { get; set; } = Vector.Zero;
         public int MaxLineNumber = 0;
 
+        public int CurrentChangeBlockIdx
+        {
+            get => _currentChangeBlockIdx;
+            set => SetProperty(ref _currentChangeBlockIdx, value);
+        }
+
         public string Repo { get; set; } = null;
         public DiffOption Option { get; set; } = null;
 
+        public List<TextDiffChangeBlock> ChangeBlocks { get; set; } = [];
+
+        public void ProcessChangeBlocks()
+        {
+            ChangeBlocks.Clear();
+            int lineIdx = 0, blockStartIdx = 0;
+            bool isNewBlock = true;
+            foreach (var line in Lines)
+            {
+                lineIdx++;
+                if (line.Type == Models.TextDiffLineType.Added ||
+                    line.Type == Models.TextDiffLineType.Deleted ||
+                    line.Type == Models.TextDiffLineType.None) // Empty
+                {
+                    if (isNewBlock)
+                    {
+                        isNewBlock = false;
+                        blockStartIdx = lineIdx;
+                    }
+                }
+                else
+                {
+                    if (!isNewBlock)
+                    {
+                        ChangeBlocks.Add(new TextDiffChangeBlock(blockStartIdx, lineIdx - 1));
+                        isNewBlock = true;
+                    }
+                }
+            }
+        }
+
         public TextDiffSelection MakeSelection(int startLine, int endLine, bool isCombined, bool isOldSide)
         {
             var rs = new TextDiffSelection();
@@ -626,6 +682,8 @@ namespace SourceGit.Models
             return true;
         }
 
+        private int _currentChangeBlockIdx = -1; // NOTE: Use -1 as "not set".
+
         [GeneratedRegex(@"^@@ \-(\d+),?\d* \+(\d+),?\d* @@")]
         private static partial Regex REG_INDICATOR();
     }