fix: correct home mask threshold after early scroll

tools/homeTopMask.test.mjs

@@ -1,6 +1,6 @@
 import test from 'node:test'
 import assert from 'node:assert/strict'
-import { getTopMaskProgress } from '../src/utils/homeTopMask.js'
+import { getTopMaskProgress, getTopMaskStartY } from '../src/utils/homeTopMask.js'
 
 test('returns 0 before the title reaches the status-bar area', () => {
   assert.equal(
@@ -28,3 +28,32 @@ test('clamps invalid values into a stable progress range', () => {
   assert.equal(getTopMaskProgress({ scrollTop: 90, startY: 80, rangeY: 0 }), 1)
   assert.equal(getTopMaskProgress({ scrollTop: Number.NaN, startY: 80, rangeY: 40 }), 0)
 })
+
+
+test('converts a viewport-relative title bottom to a document-relative start Y', () => {
+  assert.equal(
+    getTopMaskStartY({ viewportBottom: 148, scrollTop: 0, statusBarHeight: 44 }),
+    96
+  )
+
+  // The same header, measured after the user has already scrolled, must keep
+  // the same document-relative threshold instead of collapsing to 0.
+  assert.equal(
+    getTopMaskStartY({ viewportBottom: 48, scrollTop: 100, statusBarHeight: 44 }),
+    96
+  )
+})
+
+test('clamps the document-relative start Y at zero', () => {
+  assert.equal(
+    getTopMaskStartY({ viewportBottom: 20, scrollTop: 0, statusBarHeight: 44 }),
+    0
+  )
+})
+
+test('treats non-finite threshold inputs as zero without producing NaN', () => {
+  assert.equal(
+    getTopMaskStartY({ viewportBottom: Number.NaN, scrollTop: 80, statusBarHeight: 44 }),
+    28
+  )
+})