🐛 Order-agnostic page-range cfi test for inverted epub.js spans

epub.js can report a page's start cfi *after* its end cfi for a
reflowable chapter wedged between fixed-layout image spreads (same
root cause as the TTS-start fix). Two range checks assumed
start <= end and silently broke on such pages:

- isSegmentOnCurrentPage: always false → during listen, TTS
  needlessly re-navigated the reader on every segment.
- openPlayer staleness check: a still-on-page stored TTS index
  could be misclassified as stale.

Extract one shared `isCFIWithinPageRange` (app/utils/epub.ts) that
tests membership without assuming bound order — two compares, no
allocation, no duplicated logic or cross-referencing comments.
No-op for normal pages where start <= end already.

Author: williamchong

app/composables/use-tts-player-modal.ts

@@ -78,8 +78,7 @@ export function useTTSPlayerModal(options: TTSPlayerOptions) {
     let isStoredIndexStale = false
     if (storedSegmentCFI && cfi && pageEndCFI) {
       try {
-        isStoredIndexStale = epubCFI.compare(storedSegmentCFI, cfi) < 0
-          || epubCFI.compare(storedSegmentCFI, pageEndCFI) > 0
+        isStoredIndexStale = !isCFIWithinPageRange(epubCFI, storedSegmentCFI, cfi, pageEndCFI)
       }
       catch {
         // Treat malformed/unsupported CFIs as stale so we fall back to the

app/pages/reader/epub.vue

@@ -538,10 +538,11 @@ let pendingTTSDisplayCfi: string | null = null
 const epubCFI = new EpubCFI()
 
 function isSegmentOnCurrentPage(segmentCfi: string): boolean {
-  if (!currentPageStartCfi.value || !currentPageEndCfi.value) return false
+  const start = currentPageStartCfi.value
+  const end = currentPageEndCfi.value
+  if (!start || !end) return false
   try {
-    return epubCFI.compare(segmentCfi, currentPageStartCfi.value) >= 0
-      && epubCFI.compare(segmentCfi, currentPageEndCfi.value) <= 0
+    return isCFIWithinPageRange(epubCFI, segmentCfi, start, end)
   }
   catch {
     return false

app/utils/epub.ts

@@ -1,5 +1,24 @@
 import { EpubCFI } from '@likecoin/epub-ts'
 
+/**
+ * Whether `cfi` lies within the page span bounded by `pageBoundA` and
+ * `pageBoundB` (inclusive), without assuming which bound is the lower one.
+ * epub.js can report a page's start cfi *after* its end cfi for a reflowable
+ * chapter wedged between fixed-layout image spreads, so the membership test
+ * must be order-agnostic. May throw on a malformed cfi (via `compare`);
+ * callers guard with try/catch.
+ */
+export function isCFIWithinPageRange(
+  epubCFI: EpubCFI,
+  cfi: string,
+  pageBoundA: string,
+  pageBoundB: string,
+): boolean {
+  const a = epubCFI.compare(cfi, pageBoundA)
+  const b = epubCFI.compare(cfi, pageBoundB)
+  return (a >= 0 && b <= 0) || (a <= 0 && b >= 0)
+}
+
 /**
  * Resolve the first text node at or after the DOM position (container, offset).
  * For element containers, `offset` indexes into childNodes per the DOM Range spec,