fix(pointer): label a touch/pen contact as the primary button so JS press handling fires

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fix(pointer): label a touch/pen contact as the primary button so JS press handling fires

Tracks #16332. This is the main counterpart of the primary-button labeling change in #16333 (which targets 0.83-stable).

Problem

On real touch hardware, finger taps on Pressable / TouchableOpacity are ignored by pointer-event-driven press machinery, while identical mouse clicks on the same element work.

Root cause

In vnext/Microsoft.ReactNative/Fabric/Composition/CompositionEventHandler.cpp, onPointerPressed maps ActiveTouch.button exclusively from PointerUpdateKind — a mouse-only concept:

default:
  activeTouch.button = -1;
  break;

A touch or pen contact has no mouse PointerUpdateKind, so it falls through to default: and gets button = -1; the derived W3C buttons bitmask then becomes 0. The dispatched pointerdown tells JS "no button is pressed", so press logic driven by pointer events discards finger contacts. Per W3C pointer-events, a touch/pen contact is the primary button: button 0, buttons 1.

Fix

After the existing switch: if the device is not a mouse and activeTouch.button fell through to a negative value, set activeTouch.button = 0 (yielding buttons = 1 while the contact is down). Mouse behaviour is unchanged, and the < 0 guard leaves untouched any non-mouse contact that somehow carried a real button mapping.

Baseline: main @ c69cf55f67f9b03f467502dac1007ac2d9ebe209. One hunk, +10 lines, no header or API change.

Scope — what this PR deliberately does not port

main already addresses the other problems #16333 covers, by different means, so this PR ports only the button-labeling change:

• The tag == -1 release leak and the stale-pointer-reuse leak were fixed on main by Fix touch event handling, improve reliability, and optimize performance #16048 (Fix touch event handling, improve reliability, and optimize performance, merged 2026-04-24). Note main dispatches a synthesized touch Cancel for these, not a touch End.
• main also cancels capture loss per-pointer and has onPointerRoutedAway.

#16333 additionally contains a cancel-all-active-touches loop in onPointerCaptureLost, an IsPrimary self-heal purge, and a stale-touch time backstop. None of those exist on main, and I have not assessed whether they are still needed there — main's at-source cleanups may subsume them. I have left them out rather than port hardening whose necessity I cannot demonstrate on this branch. Happy to do that assessment as a follow-up if it would be useful.

Validation

This exact logic — identical condition and assignment, the only textual difference being that this PR spells the enum via the Composition::Input:: alias main already uses in this function — is running in production as part of #16333 on RNW 0.83.2: compiled into Microsoft.ReactNative from source (UseExperimentalNuget=false), x64 Release, new-arch app (Facilitron FIT), on a physical Surface-class touch tablet. There, finger taps on press targets went from ignored to firing, matching mouse clicks. Validated with real PT_TOUCH injection (InitializeTouchInjection / InjectTouchInput), not simulated mouse input — the mouse path does not exercise the touch pipeline at all.

Diff verified against the pinned base: exactly one hunk, +10 lines, LF endings preserved, and the file's current tip is byte-identical to that base (no drift).

Not verified — please weigh before merging:

• No CI has run on this branch. Azure Pipelines here is gated behind a maintainer comment, so I cannot start it; a /azp run would be appreciated.
• I have not built main with this change, nor exercised it on a device on main. The behavioural evidence above comes from the 0.83.2 app.
• Lint/format gates are unrun locally. The added block was written to match the file's existing style (the Composition::Input:: alias used elsewhere in the function; the condition line is 117 columns, within this repo's ColumnLimit: 120) — by inspection, not by running the tools.
• That main's surrounding code is materially identical to the 0.83 code this was validated on rests on reading main's CompositionEventHandler.cpp at the baseline commit (same switch, same default: button = -1), not on differential testing on main.

Caveats for reviewers:

• Change file uses type: prerelease to match the convention in main's pending change files; happy to adjust.
• The inserted comment narrates the bug in past tense (carried over verbatim from fix(fabric): touch-input reliability — primary-button labeling, guaranteed touch END/CANCEL, phantom active-touch self-heal #16333's wording). Say the word if you would prefer something terser and present-tense.

Microsoft Reviewers: Open in CodeFlow

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