fix(tui): pin the header box width so the border survives the first paint

[ericleepi314]

Follow-up to #769. The right border was still missing in a real terminal — and only appeared after resizing the window. That resize clue is what cracked it.

What was actually wrong

#769 fixed the width the panel is told, but also removed the box's explicit width, letting it size to its container. The arithmetic was never the problem — the container was.

Instrumented the component and ran the real TUI on a pty at 200 columns:

frame 1: {"term":200,"maxWidth":196,"cols":196,"leftW":44,"w":145,"wide":true}
          → box should be 196.  Rendered: 198.

On the first paint the transcript's ScrollBox has not settled: it reports its full width, without the scrollbar gutter its steady-state layout reserves. A container-sized box therefore renders two columns too wide and gets clipped. And because the intro row is committed to scrollback and never repainted, it stays clipped — until a resize forces a relayout. Hence "I have to resize the window to see the border."

Same pty run, before and after a resize:

BEFORE (200 cols)   w=198  ╭─── clawcodex v1.3.0 ──────────…────────   ← no ╮
AFTER  (190 cols)   w=187  ╭─── clawcodex v1.3.0 ──────────…───────╮   ← closes

Fix

Restore width={cols}. cols derives from transcriptPanelWidth and process.stdout.columns — both correct on frame 1 — so pinning to it is right at first paint and after resize. The same pty run now closes the box at both sizes.

#769's other half stands and is load-bearing. Without its maxWidth fix, cols would still be the composer's budget and pinning to it would reproduce the original bug. The two changes are complementary; only the second half of #769 was the wrong lever.

Why the test didn't catch it

It asserted the box fills its container — and under a correctly-sized container, "fills the container" and "holds its own width" are indistinguishable. The test only ever built a correct container, so both behaviors passed.

It now renders against a deliberately oversized container (gutter omitted, reproducing frame 1) and asserts the box holds its steady-state width regardless.

Change	Result
Remove width={cols} (the #769 state)	9 tests fail — one per width

Testing

• Verified end-to-end on a pty against the real installed TUI, before and after resize, at 200 → 190 columns.
• npm run typecheck clean; eslint clean on both changed files.
• Suite 8 failed / 1581 passed — same 8 pre-existing failures (createGatewayEventHandler, cursorDriftRegression, statusRule, useConfigSync, virtualHeights).

🤖 Generated with Claude Code

[github-actions]

Test Results

    1 files      1 suites   7m 42s ⏱️
9 065 tests 9 059 ✅ 6 💤 0 ❌
9 311 runs  9 305 ✅ 6 💤 0 ❌

Results for commit 843b228.