The old approach used document.execCommand("insertText") to directly
manipulate the contentEditable DOM, but MDXEditor (Lexical) reverted
these changes during reconciliation causing the "blink" bug.
Fix: work at the markdown string level instead — find the @query in
the markdown, replace it with @Name, and update via setMarkdown().
Also add an input event listener alongside selectionchange for more
reliable mention detection (fixes space-to-dismiss).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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