test: add home top mask progress helper

src/utils/homeTopMask.js

@@ -0,0 +1,11 @@
+export function getTopMaskProgress({ scrollTop, startY, rangeY }) {
+  const safeScrollTop = Number.isFinite(scrollTop) ? scrollTop : 0
+  const start = Math.max(0, Number.isFinite(startY) ? startY : 0)
+  const range = Math.max(1, Number.isFinite(rangeY) ? rangeY : 1)
+
+  if (safeScrollTop <= start) {
+    return 0
+  }
+
+  return Math.min(1, (safeScrollTop - start) / range)
+}