fix: Pixel-snap WrapPanel layout to prevent spurious line wraps at non-integer scales

[Xiaoy312]

GitHub Issue: re: https://github.com/unoplatform/kahua-private/issues/427

PR Type:

🐞 Bugfix

What is the current behavior? 🤔

WrapPanel accumulates child sizes as raw double values. At non-integer display scales (e.g. 1.25×, 1.5×), floating-point rounding errors cause the accumulated line width to slightly exceed the available width, triggering spurious line breaks and producing an incorrect layout.

What is the new behavior? 🚀

Child sizes and accumulated direct-axis offsets are snapped to physical pixel boundaries (via LayoutHelper.LayoutRound) in both MeasureOverride and ArrangeOverride. A half-DIP tolerance is added to line-overflow comparisons so that pixel-boundary rounding noise does not falsely trigger a new line. An XML doc comment was also added to LayoutHelper.LayoutRound to clarify its purpose.

PR Checklist ✅

Please check if your PR fulfills the following requirements:

• 📝 Commits must be following the Conventional Commits specification.
• 🧪 Added Runtime tests, UI tests, or a manual test sample for the changes have been added (for bug fixes / features) (if applicable)
• 📚 Docs have been added/updated which fit documentation template (for bug fixes / features)
• 🖼️ Validated PR Screenshots Compare Test Run results.
• ❗ Contains NO breaking changes

Other information ℹ️

Runtime tests added in Given_WrapPanel.cs covering wrap behavior at integer and non-integer scales.

[Copilot]

Pull request overview

This pull request fixes a bug in WrapPanel where floating-point rounding errors at non-integer display scales (e.g., 1.25×, 1.5×) cause spurious line wraps. The fix applies pixel-boundary snapping to child sizes and accumulated offsets using the new LayoutHelper.LayoutRound method, and adds a half-DIP tolerance to line-overflow comparisons.

Changes:

• Added LayoutHelper.LayoutRound() method with XML documentation to snap values to physical pixel boundaries
• Applied pixel snapping to child sizes and accumulated offsets in both MeasureOverride and ArrangeOverride
• Added half-DIP tolerance to line-overflow comparisons to prevent rounding noise from triggering line wraps
• Refactored variable naming from o to orientation for improved clarity
• Updated ArrangeLine method signature to accept and use the scale parameter
• Added runtime test for WrapPanel wrapping behavior at non-integer scales (1.25×)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Uno.UI/UI/LayoutHelper.cs	Added LayoutRound helper method with XML documentation for pixel-boundary snapping
src/Uno.UI/UI/Xaml/Controls/WrapPanel/WrapPanel.Layout.cs	Applied pixel snapping to layout calculations in MeasureOverride and ArrangeOverride; updated variable names and ArrangeLine signature
src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_WrapPanel.cs	Added regression test for wrap behavior at non-integer scales

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[Xiaoy312]

the linked issue is fixed by another PR in unoplatform/Uno.WindowsCommunityToolkit/pull/235
this PR addresses a similar problem found within WrapPanel

[unodevops]

🤖 Your WebAssembly Skia Sample App stage site is ready! Visit it here: https://unowasmprstaging.z20.web.core.windows.net/pr-22963/wasm-skia-net9/index.html

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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[unodevops]

🤖 Your WebAssembly Skia Sample App stage site is ready! Visit it here: https://unowasmprstaging.z20.web.core.windows.net/pr-22963/wasm-skia-net9/index.html

[unodevops]

⚠️⚠️ The build 205549 has failed on Uno.UI - CI.

[unodevops]

🤖 Your WebAssembly Skia Sample App stage site is ready! Visit it here: https://unowasmprstaging.z20.web.core.windows.net/pr-22963/wasm-skia-net9/index.html