Affinity Redesign Plan

Goal

Make affinity semantics cheap by default.

Current read

• semantic behavior is correct but split between editor transforms and always-on leaf DOM
• the expensive part is paid by every hard-affinity leaf even though the feature matters only at the active boundary

Target architecture

1. keep affinity semantics in the editor engine
2. move hard-edge DOM from always-on leaf wrappers to active-boundary-only rendering
3. keep public plugin config unchanged
4. verify with saved before/after code benchmarks and affinity behavior tests

Execution

1. freeze before benchmarks for code census + code direct leaf + code full pipe
2. add behavior tests for active-boundary affinity semantics
3. introduce editor-level affinity boundary state
4. render boundary DOM only for the active boundary
5. rerun benchmarks
6. keep or revert based on measured gain and behavior parity

Risks

• mouse boundary placement
• IME / browser-specific caret quirks
• snapshot churn from removing always-on spacer nodes

Exit criteria

• hard-affinity tests still pass
• bun check passes
• code lane improves materially versus saved before artifacts