fix(LeftSidebar): fix misbehaving mention button

[DorraJaouad]

☑️ Resolves

• Fix [Bug] 'Unread mentions' button misbehaves with conversation tags listing #18035

Before, the last layer was filtering. Now the last layer is sorting, which should be used in calculating the mentions and other visual validation.

AI (if applicable)

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

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After
Screenshot before	Screenshot after

🚧 Tasks

• ...

🏁 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

[Antreesy]

getFirstItemInViewportIndex, getLastItemInViewportIndex, getItemOffset are not taking into account ConversationTagHeader items: they are rendered and taking height, but not considered when you counting unreadMentionIndices. That's why scroll to bottom, and computing when to show a button has slightly offset with tags used.

Otherwise logic seems good

[DorraJaouad]

getFirstItemInViewportIndex, getLastItemInViewportIndex, getItemOffset are not taking into account ConversationTagHeader items: they are rendered and taking height, but not considered when you counting unreadMentionIndices. That's why scroll to bottom, and computing when to show a button has slightly offset with tags used.

Otherwise logic seems good

Will be done in a follow up

[Antreesy]

Seeing this while trying to click 'Unread mentions', otherwise OK

UPD: scroll handling is already covered by @scroll="debounceHandleScroll"