[riottt]

Summary:

Backport #59207 to 0.88-stable and add five regression tests. The production hunk is unchanged from the upstream implementation by Gregory Moskaliuk (@hryhoriiK97).

The default clear NSBackgroundColorAttributeName suppresses the Japanese IME composition underline even while the native marked range remains active. Omitting that attribute restores the underline. Visible backgrounds and foreground/background opacity remain intact. Input delegates and selection/synchronization logic are unchanged.

Related to #48497 and the underline portion of #56082.

Changelog:

[IOS] [FIXED] - Avoid a default transparent text background that can suppress the Japanese IME composition underline.

Test Plan:

RNTester and React Native Core built from source, with RCT_USE_PREBUILT_RNCORE=0. Xcode 27.0, iPhone 17 simulator / iOS 27.0, Apple Japanese kana keyboard.

• Real converter tests: new cases give 4 failed / 1 passed before → 5/5 passed after. The full test class passes 29/29. The existing test file was registered locally in RNTesterUnitTests; verification-only project edits are excluded from this PR.
• Fabric UI: 9/9 methods pass on each arm. Eight input scenarios cover empty/existing text, single/multiline, and controlled/uncontrolled values. Underlines are visible in 0/8 before → 8/8 after. Native marked range, text, selection, and JS values are asserted through composition, blur-to-commit, refocus, append, and deletion. Committed after frames contain no pixels matching the blue indicator color. Light/dark background and opacity samples match exactly. Screenshots were inspected.
• Repository-wide yarn lint, clang-format --dry-run --Werror on both changed files, and git diff --check pass.

Reproduction, source/binary hashes, XCTest summaries, and raw screenshots

Physical devices, other iOS versions/keyboards, candidate selection/reconversion, and Grok itself remain untested. This is not a fix for every Japanese/CJK input issue. Full monorepo JS tests/build were not run; dependency audit reports pre-existing findings, with no dependency changes in this PR. Details are in the linked verification notes.

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