Add OverlayPortal.overlayChildLayoutBuilder sample

[loic-sharma]

This mirrors the existing OverlayPortal example:

https://github.com/flutter/flutter/blob/master/examples/api/lib/widgets/overlay/overlay_portal.0.dart#L1-L58

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[gemini-code-assist]

Code Review

This pull request introduces a new code sample demonstrating OverlayPortal.overlayChildLayoutBuilder along with its corresponding widget tests. Feedback on the changes includes fixing a syntax error where EdgeInsets is missing from the padding property, and refactoring the tests to use more specific ancestor finders instead of generic type finders to prevent fragility.

[LongCatIsLooong]

lgtm with nits.

[LongCatIsLooong]

This offset can be NaN if the transform is a singular matrix. I think the best practice would be checking for NaN / check the determinant of the matrix (although in this example the transform is guaranteed to be a rigid motion, so maybe as a comment?)

[loic-sharma]

Added a comment! Please feel free to leave feedback on the comment, my matrix math is veeeery rusty 😂

[LongCatIsLooong]

It's probably better to transform the whole child rect sine childSize is reported in the child's own coordinates.

finalRect childRectInOverlay =MatrixUtils.transformRect(transform, Offset.zero & childSize);

The easiest way to implement follow is probably Transform but I think if there's scaling in the transform then RenderBox hit-testing culling might become a problem.

[loic-sharma]

Ah thanks, good catch! I updated this to transform the whole child rect, but stuck to Positioned instead of Transform 👍

[gemini-code-assist]

Code Review

This pull request introduces a new DartPad code sample and corresponding tests for OverlayPortal.overlayChildLayoutBuilder. A critical syntax error was identified in the code sample where the EdgeInsets class name was omitted from the padding property.

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