Repaint only when the selection changes

Source/HtmlRenderer/Core/Handlers/SelectionHandler.cs

@@ -449,6 +449,11 @@ private void HandleSelection(RControl control, RPoint loc, bool allowPartialSele
                 // if there is matching word
                 if (word != null)
                 {
+                    var previousStart = _selectionStart;
+                    var previousEnd = _selectionEnd;
+                    var previousStartIndex = _selectionStartIndex;
+                    var previousEndIndex = _selectionEndIndex;
+
                     if (_selectionStart == null)
                     {
                         // on start set the selection start word
@@ -476,7 +481,14 @@ private void HandleSelection(RControl control, RPoint loc, bool allowPartialSele
 
                     _cursorChanged = true;
                     control.SetCursorIBeam();
-                    control.Invalidate();
+
+                    // Most mouse moves stay within the same character, repainting the whole control for
+                    // them only adds flicker, especially over remote desktop.
+                    if (_selectionStart != previousStart || _selectionEnd != previousEnd
+                        || _selectionStartIndex != previousStartIndex || _selectionEndIndex != previousEndIndex)
+                    {
+                        control.Invalidate();
+                    }
                 }
             }
         }

Source/Test/HtmlRenderer.IntegrationTest/Text/SelectionInvalidateIntegrationTests.cs

@@ -0,0 +1,71 @@
+using HtmlRenderer.IntegrationTest.TestSupport;
+using TheArtOfDev.HtmlRenderer.Adapters;
+using TheArtOfDev.HtmlRenderer.Adapters.Entities;
+using TheArtOfDev.HtmlRenderer.Core.Dom;
+
+namespace HtmlRenderer.IntegrationTest.Text;
+
+/// <summary>
+/// Dragging a selection repaints the control only when the selection changes, every extra repaint shows up
+/// as flicker over remote desktop (#271).
+/// </summary>
+[DoNotParallelize]
+[TestClass]
+public sealed class SelectionInvalidateIntegrationTests
+{
+    [TestMethod]
+    public void MouseMove_WithinTheSameCharacter_DoesNotRepaint()
+    {
+        var (root, container) = LayoutHarness.Layout(LayoutHarness.Wrap("<pre id='p'>first second third</pre>"));
+        var words = new List<CssRect>();
+        CollectWords(LayoutHarness.FindById(root, "p")!, words);
+        var first = words.First(w => w.Text == "first");
+        var third = words.First(w => w.Text == "third");
+        var control = new FakeControl(container.Adapter);
+
+        container.HandleMouseDown(control, new RPoint(first.Left + 1, first.Top + 2));
+        control.Invalidations = 0;
+        var target = new RPoint(third.Left + third.Width / 2, third.Top + 2);
+
+        container.HandleMouseMove(control, target);
+        Assert.AreEqual(1, control.Invalidations, "starting the selection must repaint");
+
+        container.HandleMouseMove(control, target);
+        container.HandleMouseMove(control, new RPoint(target.X + 0.1, target.Y + 1));
+        Assert.AreEqual(1, control.Invalidations, "moving within the same character must not repaint");
+
+        container.HandleMouseMove(control, new RPoint(third.Right - 1, target.Y));
+        Assert.AreEqual(2, control.Invalidations, "extending the selection must repaint");
+    }
+
+    private static void CollectWords(CssBox box, List<CssRect> words)
+    {
+        words.AddRange(box.Words);
+        foreach (var child in box.Boxes)
+            CollectWords(child, words);
+    }
+
+    /// <summary>A control with the left mouse button held down that counts its repaints.</summary>
+    private sealed class FakeControl(RAdapter adapter) : RControl(adapter)
+    {
+        private const double CharWidth = 8;
+
+        public int Invalidations { get; set; }
+
+        public override bool LeftMouseButton => true;
+        public override bool RightMouseButton => false;
+        public override RPoint MouseLocation => RPoint.Empty;
+        public override void SetCursorDefault() { }
+        public override void SetCursorHand() { }
+        public override void SetCursorIBeam() { }
+        public override void DoDragDropCopy(object dragDropData) { }
+
+        public override void MeasureString(string str, RFont font, double maxWidth, out int charFit, out double charFitWidth)
+        {
+            charFit = Math.Min(str.Length, (int)(maxWidth / CharWidth));
+            charFitWidth = charFit * CharWidth;
+        }
+
+        public override void Invalidate() => Invalidations++;
+    }
+}