Paint full visual-line box, eliminating inter-line gaps

The two background renderers used
BackgroundGeometryBuilder.GetRectsFromVisualSegment(textView,
visualLine, 0, int.MaxValue) to enumerate the text-segment rectangles
for each visual line. Those rectangles are sized to the text-segment
bounding box, which on some DPI / line-spacing combinations does not
cover the full vertical extent of the line box -- leaving a 1-2 px
sliver of untinted background between adjacent diff-tinted rows. With
the soft tints those slivers were effectively invisible; with the new
strong full-line tints they were obvious horizontal "stripes" between
every row in a delete or insert block.

Switch both renderers to paint a single rectangle per visual line at
(0, VisualTop - ScrollOffset.Y) sized (ActualWidth, Height).
VisualLine.Height is the height of the entire line box, including
all wrapped sub-rows for the document line, so this also correctly
handles word-wrap (one DocumentLine that spans multiple visual rows
gets one rectangle covering all of them, which is the same behavior
the segment-rect loop had).

No tests added for the rendering itself -- exercising AvalonEdit's
VisualLine.VisualTop / Height accurately would require spinning up a
real TextView with measured layout, which the test project
deliberately avoids. The brush-selection logic (the only piece worth
unit-testing here) is already covered by
LineBackgroundBrushSelectorTests.

AI-Local-Session: 4519f6b6-393a-4476-8efa-410e5396c3a9
AI-Cloud-Session: 72f9e474-60ab-42c2-b2a0-28fee827cbbb
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Geeven Singh

CHANGELOG.md

@@ -30,6 +30,16 @@ body. Keep section headings exact and write notes in Markdown.
 
 ### Fixed
 
+- **Horizontal gaps between adjacent diff-tinted lines are gone.**
+  Both background renderers used to paint each line's tint from
+  `BackgroundGeometryBuilder.GetRectsFromVisualSegment`, which
+  returns rectangles sized to the text-segment bounding box. On some
+  DPI / line-spacing combinations that left a 1–2 px sliver of
+  untinted background between adjacent rows. With the soft tints
+  those slivers were invisible; with the new strong full-line tints
+  they were obvious. The renderers now paint from `VisualLine.VisualTop`
+  for the full `VisualLine.Height`, which is the actual line-box
+  height (and handles word-wrap correctly).
 - **Paired delete+insert lines no longer render as yellow "Modified"
   when intra-line diff is off.** The side-by-side renderer used to
   treat any positionally-paired delete/insert as `Modified` (yellow

DiffViewer/Rendering/DiffBackgroundRenderer.cs

@@ -65,18 +65,19 @@ public void Draw(TextView textView, DrawingContext drawingContext)
 
             Brush brush = LineBackgroundBrushSelector.Pick(_side, highlight.Kind, _scheme);
 
-            // Paint the full line width so the tint extends to the right edge
-            // even on short lines; matches GitHub / VS Code line-diff style.
-            foreach (var rect in BackgroundGeometryBuilder.GetRectsFromVisualSegment(
-                textView, visualLine, 0, int.MaxValue))
-            {
-                var fullWidth = new Rect(
-                    rect.X,
-                    rect.Y,
-                    Math.Max(rect.Width, textView.ActualWidth - rect.X),
-                    rect.Height);
-                drawingContext.DrawRectangle(brush, null, fullWidth);
-            }
+            // Paint the full visual-line box (VisualTop + Height) rather
+            // than the text-segment rect from BackgroundGeometryBuilder:
+            // the segment-rect approach leaves 1-2px horizontal gaps
+            // between adjacent lines on some DPI / line-spacing settings.
+            // VisualLine.Height covers all wrapped sub-rows for the
+            // document line, so this works for word-wrap too. The full
+            // ActualWidth ensures the tint extends to the right edge on
+            // short lines (matches GitHub / VS Code line-diff style).
+            double y = visualLine.VisualTop - textView.ScrollOffset.Y;
+            drawingContext.DrawRectangle(
+                brush,
+                pen: null,
+                new Rect(0, y, textView.ActualWidth, visualLine.Height));
         }
     }
 }

DiffViewer/Rendering/InlineDiffBackgroundRenderer.cs

@@ -41,16 +41,18 @@ public void Draw(TextView textView, DrawingContext drawingContext)
 
             Brush brush = LineBackgroundBrushSelector.Pick(DiffSide.Inline, highlight.Kind, _scheme);
 
-            foreach (var rect in BackgroundGeometryBuilder.GetRectsFromVisualSegment(
-                textView, visualLine, 0, int.MaxValue))
-            {
-                var fullWidth = new Rect(
-                    rect.X,
-                    rect.Y,
-                    Math.Max(rect.Width, textView.ActualWidth - rect.X),
-                    rect.Height);
-                drawingContext.DrawRectangle(brush, null, fullWidth);
-            }
+            // Paint the full visual-line box (VisualTop + Height) rather
+            // than the text-segment rect from BackgroundGeometryBuilder:
+            // the segment-rect approach leaves 1-2px horizontal gaps
+            // between adjacent lines on some DPI / line-spacing settings.
+            // VisualLine.Height covers all wrapped sub-rows for the
+            // document line, so this works for word-wrap too. See the
+            // matching comment in DiffBackgroundRenderer.Draw.
+            double y = visualLine.VisualTop - textView.ScrollOffset.Y;
+            drawingContext.DrawRectangle(
+                brush,
+                pen: null,
+                new Rect(0, y, textView.ActualWidth, visualLine.Height));
         }
     }
 }