Add footnote handler

footnote.js

@@ -0,0 +1,66 @@
+const refTypes = ['annoref', 'biblioref', 'glossref', 'noteref']
+const refRoles = ['doc-biblioref', 'doc-glossref', 'doc-noteref']
+const isFootnote = a => {
+    const types = a.getAttributeNS('http://www.idpf.org/2007/ops', 'type')?.split(' ')
+    const roles = a.getAttribute('role')?.split(' ')
+    return {
+        yes: types?.some(t => refTypes.includes(t)) || roles?.some(r => refRoles.includes(r)),
+        maybe: () => !types?.includes('backlink') && !roles?.includes('doc-backlink')
+            && (getComputedStyle(a).verticalAlign === 'super'
+            || a.children.length === 1 && getComputedStyle(a.children[0]).verticalAlign === 'super')
+            || getComputedStyle(a.parentElement).verticalAlign === 'super',
+    }
+}
+
+const isInline = 'a, span, sup, sub, em, strong, i, b, small, big'
+const extractFootnote = (doc, anchor) => {
+    let el = anchor(doc)
+    while (el.matches(isInline)) {
+        const parent = el.parentElement
+        if (!parent) break
+        el = parent
+    }
+    return el
+}
+
+export class FootnoteHandler extends EventTarget {
+    detectFootnotes = true
+    #showFragment(book, { index, anchor }, getHref) {
+        const view = document.createElement('foliate-view')
+        view.addEventListener('load', e => {
+            const { doc } = e.detail
+            const el = anchor(doc)
+            if (el) {
+                const range = el.startContainer ? el : doc.createRange()
+                if (!el.startContainer) {
+                    if (el.matches('li, aside')) range.selectNodeContents(el)
+                    else range.selectNode(el)
+                }
+                const frag = range.extractContents()
+                doc.body.replaceChildren()
+                doc.body.appendChild(frag)
+            }
+            const detail = { view, href: getHref(el) }
+            this.dispatchEvent(new CustomEvent('render', { detail }))
+        })
+        view.open(book)
+            .then(() => this.dispatchEvent(new CustomEvent('before-render', { detail: { view } })))
+            .then(() => view.goTo(index))
+    }
+    handle(book, e) {
+        const { a, href } = e.detail
+        const { yes, maybe } = isFootnote(a)
+        if (yes) {
+            e.preventDefault()
+            Promise.resolve(book.resolveHref(href)).then(target =>
+                this.#showFragment(book, target, el => el?.matches?.('aside') ? null : href))
+        }
+        else if (this.detectFootnotes && maybe()) {
+            e.preventDefault()
+            Promise.resolve(book.resolveHref(href)).then(({ index, anchor }) => {
+                const target = { index, anchor: doc => extractFootnote(doc, anchor) }
+                this.#showFragment(book, target, () => href)
+            })
+        }
+    }
+}