fix: glossary tooltip layout and viewport overflow

linkify() was returning a bare array, so its fragments landed as
direct children of flex <li> bullets — each text/link piece became
its own flex item and scrambled the reading order on wrap. Now
wrapped in a single <span> so text flows normally.

Tooltips also always centered under the term regardless of position,
overflowing off-screen near the viewport edges. GlossaryMark now
measures the anchor on hover and flips to a left- or right-aligned
placement when centering would push it out of view.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/components/GlossaryMark.tsx

@@ -1,27 +1,58 @@
-import type { ReactNode } from 'react'
+import { useRef, useState, type ReactNode } from 'react'
 import { Link } from 'react-router-dom'
 import { useLang } from '../context/LangContext'
 import { GLOSSARY } from '../glossary/terms'
 
 const BY_ID = new Map(GLOSSARY.map((entry) => [entry.id, entry]))
 
+const TOOLTIP_WIDTH = 256 // px, matches w-64
+const VIEWPORT_MARGIN = 16 // px, keep clear of the screen edge
+
+type Placement = 'start' | 'center' | 'end'
+
+const PLACEMENT_CLASS: Record<Placement, string> = {
+  start: 'left-0',
+  center: 'left-1/2 -translate-x-1/2',
+  end: 'right-0',
+}
+
 export default function GlossaryMark({ id, children }: { id: string; children: ReactNode }) {
   const { lang } = useLang()
   const entry = BY_ID.get(id)
+  const anchorRef = useRef<HTMLAnchorElement>(null)
+  const [placement, setPlacement] = useState<Placement>('center')
+
   if (!entry) return <>{children}</>
 
   const name = entry.fullName ? entry.fullName[lang] : entry.display
 
+  function handleEnter() {
+    const rect = anchorRef.current?.getBoundingClientRect()
+    if (!rect) return
+    const centerX = rect.left + rect.width / 2
+    const half = TOOLTIP_WIDTH / 2
+    if (centerX - half < VIEWPORT_MARGIN) {
+      setPlacement('start')
+    } else if (centerX + half > window.innerWidth - VIEWPORT_MARGIN) {
+      setPlacement('end')
+    } else {
+      setPlacement('center')
+    }
+  }
+
   return (
     <Link
+      ref={anchorRef}
       to={`/glossary#${entry.id}`}
+      onMouseEnter={handleEnter}
       className="group relative border-b border-dotted border-sky-500/60 hover:border-sky-400 hover:text-sky-300 transition-colors"
     >
       {children}
       <span
-        className="pointer-events-none absolute bottom-full left-1/2 z-20 mb-2 w-64 -translate-x-1/2 rounded-lg
+        className={`pointer-events-none absolute bottom-full z-20 mb-2 w-64 rounded-lg
                    border border-slate-700 bg-slate-900 p-3 text-left text-xs font-normal normal-case
-                   text-slate-400 opacity-0 shadow-2xl transition-opacity group-hover:opacity-100"
+                   text-slate-400 opacity-0 shadow-2xl transition-opacity group-hover:opacity-100
+                   ${PLACEMENT_CLASS[placement]}`}
       >
         <span className="mb-1 block font-semibold text-slate-200">{name}</span>
         {entry.definition[lang]}

src/lib/glossary.tsx

@@ -16,12 +16,17 @@ for (const entry of GLOSSARY) {
 const ALL_SURFACES = [...MATCH_TO_ID.keys()].sort((a, b) => b.length - a.length)
 const GLOSSARY_REGEX = new RegExp(`\\b(${ALL_SURFACES.map(escapeRegExp).join('|')})\\b`, 'g')
 
-/** Wraps every occurrence of a known glossary term in `text` with a hoverable, linked <GlossaryMark>. */
+/** Wraps every occurrence of a known glossary term in `text` with a hoverable, linked <GlossaryMark>.
+ *  Always returns a single inline element — never a bare array — so callers can safely place the
+ *  result inside a `flex` container (e.g. a bulleted `<li>`) without each fragment becoming its own
+ *  flex item and breaking the reading order. */
 export function linkify(text: string): React.ReactNode {
   const parts = text.split(GLOSSARY_REGEX)
   if (parts.length === 1) return text
 
-  return parts.map((part, i) => {
+  return (
+    <span>
+      {parts.map((part, i) => {
         const id = MATCH_TO_ID.get(part)
         if (id === undefined) return <Fragment key={i}>{part}</Fragment>
         return (
@@ -29,5 +34,7 @@ export function linkify(text: string): React.ReactNode {
             {part}
           </GlossaryMark>
         )
-  })
+      })}
+    </span>
+  )
 }