Checkpoint web parity through Chromium input tasks

web/protocol/viewport-dpr.ts

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+export const VIEWPORT_DPR_SCHEMA_VERSION = 1 as const;
+export const VIEWPORT_MAX_DPR = 2 as const;
+
+export interface ViewportPixelMetrics {
+  schemaVersion: typeof VIEWPORT_DPR_SCHEMA_VERSION;
+  cssWidth: number;
+  cssHeight: number;
+  pixelRatio: number;
+  backingWidth: number;
+  backingHeight: number;
+}
+
+export interface ViewportNDC {
+  x: number;
+  y: number;
+}
+
+function finitePositive(value: number): boolean {
+  return Number.isFinite(value) && value > 0;
+}
+
+export function resolveViewportPixelRatio(devicePixelRatio: number | undefined, maximum = VIEWPORT_MAX_DPR): number {
+  if (!finitePositive(maximum)) throw new Error("VIEWPORT_DPR_INVALID");
+  const observed = finitePositive(devicePixelRatio ?? 1) ? devicePixelRatio! : 1;
+  return Math.min(observed, maximum);
+}
+
+export function resolveViewportPixelMetrics(cssWidth: number, cssHeight: number, devicePixelRatio: number | undefined, maximum = VIEWPORT_MAX_DPR): ViewportPixelMetrics {
+  if (!finitePositive(cssWidth) || !finitePositive(cssHeight)) throw new Error("VIEWPORT_SIZE_INVALID");
+  const pixelRatio = resolveViewportPixelRatio(devicePixelRatio, maximum);
+  return {
+    schemaVersion: VIEWPORT_DPR_SCHEMA_VERSION,
+    cssWidth,
+    cssHeight,
+    pixelRatio,
+    backingWidth: Math.max(1, Math.floor(cssWidth * pixelRatio)),
+    backingHeight: Math.max(1, Math.floor(cssHeight * pixelRatio)),
+  };
+}
+
+export function viewportNDC(clientX: number, clientY: number, bounds: { left: number; top: number; width: number; height: number }): ViewportNDC {
+  if (![clientX, clientY, bounds.left, bounds.top, bounds.width, bounds.height].every(Number.isFinite) || bounds.width <= 0 || bounds.height <= 0) {
+    throw new Error("VIEWPORT_BOUNDS_INVALID");
+  }
+  return {
+    x: ((clientX - bounds.left) / bounds.width) * 2 - 1,
+    y: -((clientY - bounds.top) / bounds.height) * 2 + 1,
+  };
+}