[antonreshetov]

Summary

• keep the insertion action centered on the line with a 24px hit area
• preserve horizontal level selection only where multiple structural scopes share a boundary
• restrict Block, Row, Cell, and Item actions to their actual tree scope boundaries
• keep empty-cell insertion and collapsed-scope behavior predictable

Verified behavior

• internal Atom boundary: Item only at every X position
• Cell boundary: Cell or Item only
• shared final Block/Row/Cell/Atom boundary: Block, Row, Cell, or Item by X position
• inserted nodes use the expected parent and index

Checks

• pnpm -s vue-tsc --noEmit
• pnpm -s eslint scoped changed files
• pnpm -s test (142 tests)