Fix MOBI anchors being calculated too late (#118)

The MOBI6.init() method builds #fileposList (the map of all filepos byte-offset references, used to insert <a id="fileposXXXX"> anchors into section HTML) after calling getGuide() and the TOC-section loader — both of which call createDocument() → loadText(), which caches the section HTML. Since #fileposList is still empty at that point, the cached HTML has no anchor elements.

Later, when Foliate calls getElementById('filepos12345') on the cached (anchor-less) document, it returns null, so the paginator falls back to ?? 0 — i.e. scroll to position 0 = first page.

For EPUB this doesn't happen because chapters are separate files; navigation loads a new section by index, not by searching for an anchor element within one section.

Co-authored-by: Lars E. <lars@ermert.es>

mobi.js

@@ -719,6 +719,16 @@ class MOBI6 {
             size: section.end - section.start,
         }))
 
+        // get list of all `filepos` references in the book,
+        // which will be used to insert anchor elements
+        // because only then can they be referenced in the DOM
+        // NOTE: must be built BEFORE getGuide()/createDocument() so that sections
+        // cached during init() already have anchor elements inserted.
+        this.#fileposList = [...new Set(
+            Array.from(str.matchAll(fileposRegex), m => m[1]))]
+            .map(filepos => ({ filepos, number: Number(filepos) }))
+            .sort((a, b) => a.number - b.number)
+
         try {
             this.landmarks = await this.getGuide()
             const tocHref = this.landmarks
@@ -765,14 +775,6 @@ class MOBI6 {
             console.warn(e)
         }
 
-        // get list of all `filepos` references in the book,
-        // which will be used to insert anchor elements
-        // because only then can they be referenced in the DOM
-        this.#fileposList = [...new Set(
-            Array.from(str.matchAll(fileposRegex), m => m[1]))]
-            .map(filepos => ({ filepos, number: Number(filepos) }))
-            .sort((a, b) => a.number - b.number)
-
         this.metadata = this.mobi.getMetadata()
         this.getCover = this.mobi.getCover.bind(this.mobi)
         return this