fix(transcript): harden chunks rendering - #19868

☑️ Resolves

• Fix TranscriptBlock: TypeError in updateLines/getLineBoundaries when a chunk has no layout boxes (empty getClientRects) #19733

Setup for all scenarios (during the call in the console):

const pinia = document.querySelector('#content').__vue_app__.config.globalProperties.$pinia
pinia._s.get('callView').isLiveTranscriptionEnabled = true
await setTimeout(() => {}, 1000)
const r = document.querySelector('.transcript').__vueParentComponent.proxy
const say = (message, peerId = 'a', final = true) => r.handleTranscript({ attributes: { peerId, actorId: peerId, actorType: 'users', name: peerId.toUpperCase() } }, message, 'en', final)
const hide = (hidden = true) => { document.querySelector('.transcript').style.display = hidden ? 'none' : '' }

Scenario 1: skip line detection for chunks without layout

• Run: say('hello'); setTimeout(() => { hide(); say('more') }, 3000)
• Before: Uncaught TypeError: Cannot read properties of undefined (reading 'lastChunkIndex') at Proxy.updateLines
• After: clean.

Scenario 2: stop line detection when the last known chunk has no layout

• Run: say('hello'); say('more'); setTimeout(() => { hide(); say('other speaker', 'b') }, 3000)
• Before: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'top') at Proxy.updateLines
• After: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'bottom') at Proxy.getLineBoundaries.

Scenario 3: return no line boundaries when lines have no layout

• Run: say('hello'); say('more'); setTimeout(() => { hide(); say('other speaker', 'b') }, 3000)
• Before: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'bottom') at Proxy.getLineBoundaries
• After: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'bottom') at Proxy.scrollToBlockAndLine.

Scenario 4: skip scrolling to a line without layout

• Run: say('hello'); say('more'); setTimeout(() => { hide(); say('other speaker', 'b') }, 3000)
• Run after a few seconds: hide(false)
• Before: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'bottom') at Proxy.scrollToBlockAndLine
• After: clean

AI (if applicable)

• The content of this PR was partly or fully generated using AI

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Integrations with Files sidebar and other apps
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required