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fix(textbox): subpixel gaps between adjacent selected characters #22962

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fix(textbox): subpixel gaps between adjacent selected characters #22962

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fe91429
fix(textbox): subpixel gaps between adjacent selected characters
ramezgerges Apr 3, 2026
e33a514
test: add When_Selection_Background_Has_No_Gaps_Between_Characters
ramezgerges Apr 3, 2026
02b39e8
chore: make same adjustment vertically
ramezgerges Apr 3, 2026
4168cab
Merge branch 'master' into textbox_selection_gaps
MartinZikmund Apr 8, 2026
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30 changes: 30 additions & 0 deletions src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_TextBox.skia.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -5104,6 +5104,36 @@
}
#endif

[TestMethod]
public async Task When_Selection_Background_Has_No_Gaps_Between_Characters()
{
Comment on lines +5107 to +5109
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Copilot AI Apr 8, 2026

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The PR description currently includes closes # without an issue number/link. Please update it to use a fully-qualified issue URL (e.g., Closes https://github.com/unoplatform/uno/issues/<id>) or note explicitly that there is no related issue and who approved it, per the repo PR requirements.

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using var _ = new TextBoxFeatureConfigDisposable();

// Text with spaces that can produce cluster boundaries at exact
// integer coordinates, which previously caused 1-pixel gaps in
// the selection background due to rounding/antialiasing issues.
var SUT = new TextBox
{
Text = "____",
FontSize = 14,
Comment on lines +5112 to +5118
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Copilot AI Apr 8, 2026

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The comment above this test says it relies on “Text with spaces”, but the actual test text is "____" (underscores). Please either update the text to match the intended reproduction case, or adjust the comment to reflect why underscores are the right input for triggering cluster-boundary rounding issues.

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Width = 300,
SelectionHighlightColor = new SolidColorBrush(Colors.Red),
};

await UITestHelper.Load(SUT);

SUT.Focus(FocusState.Programmatic);
SUT.SelectAll();
await UITestHelper.WaitForIdle();

var tb = SUT.FindVisualChildByType<TextBlock>();
var screenshot = await UITestHelper.ScreenShot(tb);
for (int i = 0; i < 20; i++)
{
screenshot.GetPixel(i, screenshot.Height / 2).Should().Be(Colors.Red, $"Selection background should have no gaps at x={i}");
Comment on lines +5131 to +5133
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The test compares pixel colors with exact equality (Should().Be(Colors.Red)), which is brittle with antialiasing and subpixel blending at highlight edges. To reduce flakiness, prefer an assertion that allows a small tolerance (or sample pixels away from edges / validate a rectangle region using ImageAssert’s tolerance helpers).

Suggested change
for (int i = 0; i < 20; i++)
{
screenshot.GetPixel(i, screenshot.Height / 2).Should().Be(Colors.Red, $"Selection background should have no gaps at x={i}");
const byte colorTolerance = 8;
for (int i = 0; i < 20; i++)
{
var pixel = screenshot.GetPixel(i, screenshot.Height / 2);
pixel.A.Should().BeInRange((byte)Math.Max(0, Colors.Red.A - colorTolerance), (byte)Math.Min(255, Colors.Red.A + colorTolerance), $"Selection background alpha should have no gaps at x={i}");
pixel.R.Should().BeInRange((byte)Math.Max(0, Colors.Red.R - colorTolerance), (byte)Math.Min(255, Colors.Red.R + colorTolerance), $"Selection background red channel should have no gaps at x={i}");
pixel.G.Should().BeInRange((byte)Math.Max(0, Colors.Red.G - colorTolerance), (byte)Math.Min(255, Colors.Red.G + colorTolerance), $"Selection background green channel should have no gaps at x={i}");
pixel.B.Should().BeInRange((byte)Math.Max(0, Colors.Red.B - colorTolerance), (byte)Math.Min(255, Colors.Red.B + colorTolerance), $"Selection background blue channel should have no gaps at x={i}");

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}
}

Check failure on line 5135 in src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_TextBox.skia.cs

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GitHub Actions / Analyze (csharp) 

Fix formatting (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/style-rules/ide0055)

Check failure on line 5135 in src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_TextBox.skia.cs

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GitHub Actions / Analyze (csharp) 

Fix formatting (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/style-rules/ide0055)
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Copilot AI Apr 8, 2026

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This assertion checks only the first 20 pixels of the screenshot, so it doesn’t actually validate “no gaps between characters” across the whole selected text (a gap could exist later and the test would still pass). It’s also sensitive to template padding/offsets because it assumes the selection background starts at x=0. Consider deriving the expected highlight span from the screenshot (e.g., find the highlight bounds first) and then asserting there are no non-highlight pixels within that span.

Suggested change
for (int i = 0; i < 20; i++)
{
screenshot.GetPixel(i, screenshot.Height / 2).Should().Be(Colors.Red, $"Selection background should have no gaps at x={i}");
}
}
var y = screenshot.Height / 2;
var startX = Enumerable.Range(0, screenshot.Width)
.FirstOrDefault(x => screenshot.GetPixel(x, y) == Colors.Red);
var endX = Enumerable.Range(0, screenshot.Width)
.LastOrDefault(x => screenshot.GetPixel(x, y) == Colors.Red);
(startX <= endX).Should().BeTrue("Selection background should be visible on the sampled scanline.");
for (int x = startX; x <= endX; x++)
{
screenshot.GetPixel(x, y).Should().Be(Colors.Red, $"Selection background should have no gaps between x={startX} and x={endX}; found a gap at x={x}");
}
}

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Copilot AI Apr 8, 2026

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Indentation for the closing brace is inconsistent here (spaces instead of the surrounding tabs). Please align it with the file’s existing indentation to avoid noisy diffs and keep formatting consistent.

Suggested change
}
}

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[TestMethod]
public async Task When_OuterScrollViewer_BringIntoView_Scrolls_To_Caret()
{
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11 changes: 6 additions & 5 deletions src/Uno.UI/UI/Xaml/Documents/UnicodeText.skia.cs
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Expand Up @@ -888,12 +888,13 @@ public void Draw(in Visual.PaintingSession session,
}
}

// The rounding is to avoid 1-pixel gaps between adjacent clusters that have a background, even with antialiasing.
// Floor every edge and +1 the trailing edges so adjacent background
// rects always overlap by 1 px, preventing antialiased-edge seams.
var backgroundRect = new SKRect(
MathF.Round(unalignedX + alignmentOffset, MidpointRounding.AwayFromZero),
y,
(float)Math.Round(unalignedX + alignmentOffset + cluster.Value.width),
y + line.lineHeight);
MathF.Floor(unalignedX + alignmentOffset),
MathF.Floor(y),
MathF.Floor(unalignedX + alignmentOffset + cluster.Value.width) + 1,
MathF.Floor(y + line.lineHeight) + 1);
highlighter.Value.background?.Paint(session.Canvas, session.Opacity, backgroundRect);

if (_corrections?[wordBoundariesIndex] is { } correction)
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