GeezerVison

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• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

On Windows, a percentage width/height is resolved by the view itself:

• Below 100%: after each SizeChanged, a deferred pass sets Width = parent.ActualWidth * pct.
• 100%: becomes Stretch alignment.

A parent that lays out its children with its own engine can't use that. The value ignores the parent's padding and the child's position in a row, and Stretch can't express 100% of a row item or a percentage height. The explicit XAML Width also overrides whatever size the parent's layout gives the child.

What is the new behavior?

The Windows View gains _setChildPercentSize(child, horizontal, fraction): boolean, which returns false by default. It's declared, Windows-only, in index.d.ts. A view with a percentage width or height hands it to its parent's _setChildPercentSize. When that returns true:

• XAML size: Width/Height stay unset.
• Core's own sizing: skipped (no size watch, no Stretch, no flex-basis percent).

Setting a length later calls it again with null to clear it. Every existing parent uses the default, so their behavior doesn't change.

Related: triniwiz/nativescript-mason#67 overrides it for its layouts.

Testing: tested on Windows x64 with that branch, covering 50% in a row, 100% × 100% in a row, 50% height with margins in a fixed-height column, and 75% of a grid cell. All resolve against the containing block. tsc -p packages/core/tsconfig.lib.json reports no errors. There are no specs for the Windows view.