Slate v2 Active Radius Policy Batch

Goal

Stop guessing the corridor default.

Measure activeRadius variants, choose the best default, and avoid dragging the runtime into more tuning theater than RC actually needs.

Measured Sweep at 10000 blocks

Radius 0

• ready 541.03ms
• type 37.95ms
• select-all 28.02ms
• paste 34.63ms
• promote 95.38ms
• promote then type 36.49ms

Radius 1

• ready 513.04ms
• type 39.35ms
• select-all 25.60ms
• paste 34.46ms
• promote 108.72ms
• promote then type 36.19ms

Radius 2

• ready 522.04ms
• type 36.03ms
• select-all 26.01ms
• paste 35.69ms
• promote 124.77ms
• promote then type 36.01ms

Decision

Set the default activeRadius to 1.

That is the best balance:

• better ready than radius 0
• better select-all than radius 0
• slightly better paste than radius 0
• better shell count posture
• much cheaper promotion than radius 2

Radius 2 buys tiny steady-state wins while making promotion meaningfully more expensive. That is not the right RC trade.

Kept Work

• default activeRadius in editable-text-blocks.tsx changed from 0 to 1
• huge-document example default query fallback in huge-document.tsx changed from 0 to 1
• islands proof lane default in replacement-huge-document-islands-benchmark.mjs changed from 0 to 1
• narrow shell runtime tests remain explicit with activeRadius: 0 so they still prove the strict shell case

Verdict

This is the point where perf work becomes optional polishing instead of RC debt.

The large-document runtime is already doing the important things right:

• far descendants do not mount by default
• broad ops stay model-driven
• promotion creates a real editing corridor
• default huge-doc lane stays green

The remaining typing gap versus legacy chunking at scale is no longer large enough to justify another architecture wave before RC.