Sidebar usage can flicker/stick at 100% on thread resume

[amanthanvi]

Summary

Entering an in-progress thread can cause the bottom-left sidebar usage bars (Session, Weekly) to briefly show correct values, then flip to 100%, sometimes recover, and then get stuck at 100%.

Reproduction

1. Enable Settings → Display → Show remaining Codex limits.
2. Open a workspace with active/in-progress thread activity.
3. Enter/switch into the in-progress thread.
4. Observe the sidebar usage bars.

Actual

• Usage values can oscillate between correct values and 100%.
• In some runs, both bars eventually remain stuck at 100%.

Expected

• Sidebar usage bars should only change when valid usage values actually change.
• Partial rate-limit updates must not overwrite known-good usage values.
• Unknown usage should render as unknown (--), not fabricated values.

Root Cause (frontend)

Rate-limit normalization currently defaults missing numeric fields to 0 and is used for both:

• explicit account/rateLimits/read refresh responses, and
• live account/rateLimits/updated events.

When incoming payloads are partial (for example, missing usedPercent / used_percent), the normalizer can coerce usage to 0. In remaining mode, that is rendered as 100% remaining, producing the observed flash/stuck behavior.

Proposed Fix

• Make rate-limit normalization merge-aware:
  • preserve previous known-good usedPercent when incoming payload omits usage values;
  • support both used-style and remaining-style percent fields;
  • avoid fabricating usage from absent fields.
• Apply merge-aware normalization in both refresh and live-event paths before reducer updates.
• Add regression tests for:
  • partial payloads preserving prior usage,
  • missing percent fields not becoming fake 100%,
  • event + refresh path behavior.

Acceptance Criteria

• No false 100% usage flashes during thread entry/resume.
• No stuck 100% state from partial rate-limit updates.
• Existing valid updates still propagate immediately.
• Targeted tests and typecheck pass.