fix(Android): use per-display density for px→dp conversion in FabricEnabledViewGroup.updateState

[ziponia]

Description

Fabric mounts views using the density of the display the surface is attached to, but FabricEnabledViewGroup.updateState converts the natively measured px size back to dp with the process-global density (PixelUtil), captured once from the device's main display.

On a display whose density differs from the main one (Samsung DeX, freeform multi-window, external monitors), every Screen pushes windowSize ÷ mainDisplayDensity into the Shadow Tree — all stack/tab content collapses into a 1/density-sized top-left box, while plain RN views outside screens track the window correctly.

Closes #4159.

Changes

• FabricEnabledViewGroup.updateState (Android, Fabric): convert px→dp using the view's own context density (context.resources.displayMetrics.density), with a defensive fallback to PixelUtil.toDIPFromPixel if the density is not positive.
• On single-display devices the context density equals the global density, so behavior there is unchanged.

Before & after - visual documentation

Before — content in a 1/density box	After — content fills the window

(The dark card is a diagnostic overlay that instruments updateState pushes. In the "after" screenshot the visible text rendering is correct because the app also applies a workaround for a separate react-native core bug — stale global density in text sp→px conversion — which is out of scope for this PR and reported to facebook/react-native.)

Measured updateState pushes on a Galaxy S25 Ultra (main display density 2.8125) in DeX, window resized 6 times:

window (dp)	pushed state before fix	pushed state after fix
394 × 702	140 × 235	394 × 662
497 × 730	177 × 245	—
737 × 794	262 × 268	—
421 × 788	150 × 266	421 × 748
338 × 727	—	338 × 687
429 × 790	—	429 × 750

Before the fix every push equals window ÷ 2.8125; after the fix it equals the window content size at every tested window size.

Test plan

• Minimal reproducer (RN 0.86, New Architecture, based on the official reproducer template): https://github.com/ziponia/rn-multidisplay-density-repro — shows live on-screen measurements (outside-stack vs inside-ScreenStack size and the window ÷ content ratio).
• Steps: yarn android on a DeX-capable device → switch to Samsung DeX (or open a freeform window on a different-density display) → inside-stack content size now matches the window, ratio ≈ 1; resize the window and verify it keeps tracking.
• Verified on a physical Galaxy S25 Ultra (SM-S938N, One UI / Android 16) across 6 window sizes; phone (single display) behavior unchanged.

Checklist

• Included code example that can be used to test this change (reproducer repo above).
• For visual changes, included screenshots / GIFs / recordings documenting the change.
• For API changes, updated relevant public types — N/A, no public API changes.
• Ensured that CI passes — pending CI on this PR.

Disclosure

This PR was authored by Claude (an AI coding agent) operating on behalf of and supervised by @ziponia, who hit this bug in a production app. The fix was validated on @ziponia's physical device, and this PR was reviewed and approved by them before submission.

[kkafar]

Hey! This patch looks good and the explanation here and in the #4159 sounds legit.

I'll check out the reproducer on Monday & if everything's good we'll land it.

Thanks!