Instead of unmounting/remounting BlockNoteTab on every tab switch
(via key={activeTabPath}), render all open tabs simultaneously and
toggle visibility with CSS display:none/block. This eliminates
editor recreation and markdown re-parsing on switch. Also adds a
loading skeleton for new tabs and uses a ref for tabs to avoid
stale closures in handleSelectNote.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
