fix(web): model picker no longer jumps at the end of its open animation

[Claudesaul]

Problem

When the model picker opens upward, its top edge jumps a few pixels as the open animation ends.

Change

The popup zooms in from scale(0.98), and LegendList's first row measurement uses getBoundingClientRect(), which includes that zoom. Each row reads about 1px short, Base UI holds the popup at that height for the animation, then it snaps to the real height. Regressed in #15152, which sizes the picker from the list's height. Our existing LegendList patch now takes that first measurement from offsetWidth/offsetHeight, which ignore transforms like ResizeObserver does. LegendList rounds sizes to whole pixels, so unscaled lists get the same sizes as before.

Scope and approval

Regression from #15152. Only LegendList's first row measurement changes.

Verification

Logged the popup's top edge every frame. Before, it settles at 528px and then snaps to 525px. After, it eases straight to 525px.

Before	After

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📝 Walkthrough

The patch updates keyboard composer inset handling and React Native list inset and end-scroll behavior. It also changes web list measurements, DOM reordering, and temporary scroll-adjustment padding.

Changes: List layout and scrolling

Layer / File(s)	Summary
Inset, scroll maintenance, and web layout adjustments patches/@legendapp__list@3.3.5.patch	The keyboard composer inset hook accepts height adjustment and animation duration. React Native lists support leading and end inset adjustments and update end-scroll maintenance. Web builds use untransformed element dimensions, a shared DOM move helper, and the applied padding value for temporary scroll adjustment.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: juliusmarminge

Merge Risk: 🔵 Low · up to c7c5b

The fix removes the model picker's jump during its open animation. Lists with fractional row heights may now see small rounding differences in their positions. Confirm that this doesn't matter for those lists before merging.

### Architecture Summary

Architecture risk: 🔵 Low · up to c7c5b

The change affects 1 system.

Changed systems: patches

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — patches (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in patches/@legendapp__list@3.3.5.patch: The react.js patch changes the recorded new-file index hash.
• observed — Modified behavior in patches/@legendapp__list@3.3.5.patch: Container layout baselines now use offsetHeight and offsetWidth instead of getBoundingClientRect, avoiding ancestor transforms in the measured dimensions.
• observed — Modified behavior in patches/@legendapp__list@3.3.5.patch: DOM reordering now routes moves before a stable element or to the end through the shared move helper instead of calling insertion methods directly.
• observed — Modified behavior in patches/@legendapp__list@3.3.5.patch: Temporary padding tracking now records the content node’s applied padding value rather than the separately constructed string.

Pre-merge checks: 4

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title is concise, uses the repository’s conventional format, and clearly describes the web model picker animation fix.
Description check		The description includes the required Problem, Change, Scope and approval, and Verification sections. It explains the cause, implementation, scope, and observed before-and-after behavior with visual e…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Inline comments:
Review comments at @patches/@legendapp__list@3.3.5.patch:
- Line 1482: Update the container measurement used to build rect so it preserves
fractional layout dimensions without being affected by transforms; replace
offsetHeight and offsetWidth with a transform-independent fractional
measurement, such as the element’s ResizeObserver border-box size. Apply the
same change to both measurement sites.

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