fix: correct home mask threshold after early scroll

src/utils/homeTopMask.js

@@ -1,3 +1,16 @@
+/**
+ * Convert a viewport-relative header bottom into a document-relative mask
+ * threshold. Selector queries report coordinates relative to the current
+ * viewport, so the scroll offset at measurement time must be added back.
+ */
+export function getTopMaskStartY({ viewportBottom, scrollTop, statusBarHeight }) {
+  const safeViewportBottom = Number.isFinite(viewportBottom) ? viewportBottom : 0
+  const safeScrollTop = Number.isFinite(scrollTop) ? scrollTop : 0
+  const safeStatusBarHeight = Number.isFinite(statusBarHeight) ? statusBarHeight : 0
+
+  return Math.max(0, safeViewportBottom + safeScrollTop - safeStatusBarHeight - 8)
+}
+
 export function getTopMaskProgress({ scrollTop, startY, rangeY }) {
   const safeScrollTop = Number.isFinite(scrollTop) ? scrollTop : 0
   const start = Math.max(0, Number.isFinite(startY) ? startY : 0)