Checkpoint web parity through Chromium input tasks

web/protocol/pointer-contract.ts

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+export const POINTER_CONTRACT_SCHEMA_VERSION = 1 as const;
+export type PointerKind = "mouse" | "touch" | "pen";
+
+export interface PointerObservation {
+  schemaVersion: typeof POINTER_CONTRACT_SCHEMA_VERSION;
+  pointerType: PointerKind;
+  pointerId: number;
+  pressure: number;
+  tiltX: number;
+  tiltY: number;
+  button: number;
+  buttons: number;
+  cancelled: boolean;
+}
+
+function bounded(value: number, min: number, max: number, fallback: number): number {
+  return Number.isFinite(value) ? Math.max(min, Math.min(max, value)) : fallback;
+}
+
+export function observePointerEvent(event: { pointerType?: string; pointerId?: number; pressure?: number; tiltX?: number; tiltY?: number; button?: number; buttons?: number; type?: string }): PointerObservation {
+  const pointerType = event.pointerType === "touch" || event.pointerType === "pen" || event.pointerType === "mouse" ? event.pointerType : null;
+  if (!pointerType) throw new Error("POINTER_TYPE_UNSUPPORTED");
+  if (!Number.isSafeInteger(event.pointerId) || event.pointerId! < 0) throw new Error("POINTER_ID_INVALID");
+  return {
+    schemaVersion: POINTER_CONTRACT_SCHEMA_VERSION,
+    pointerType,
+    pointerId: event.pointerId!,
+    pressure: bounded(event.pressure ?? (pointerType === "mouse" ? 0 : 0.5), 0, 1, 0),
+    tiltX: bounded(event.tiltX ?? 0, -90, 90, 0),
+    tiltY: bounded(event.tiltY ?? 0, -90, 90, 0),
+    button: Number.isInteger(event.button) ? event.button! : -1,
+    buttons: Number.isInteger(event.buttons) && event.buttons! >= 0 ? event.buttons! : 0,
+    cancelled: event.type === "pointercancel",
+  };
+}