[BUG] Pane resize during a running turn leaves a blank band and stale/missing rows in the live viewport (Herdr, macOS)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Code runs inside a pane of the Herdr terminal (macOS, TERM_PROGRAM=herdr). While a long turn is in progress, resizing the pane leaves the live viewport (not just scrollback) in a corrupted state that persists until the next full redraw. Two symptoms, both visible in the attached screenshots:

1. A large blank band appears inside the transcript.
After a Write(...) tool call whose result shows a 10-line file preview and a … +647 lines footer, there is a block of 6–7 completely empty rows before the └ Allowed by auto mode classifier line. In the second screenshot the same gap is still there several minutes later, and the next Write(...) tool result below it renders correctly with no gap, so the band is not padding the UI intends to draw. It looks like rows that were reserved at the old width and never reclaimed or repainted at the new width.

2. Rows inside the tool-result preview are stale or missing.
In the same file preview:

• Line 3 wraps as ... Steps us followed by a row that starts e checkbox (- [ ]) syntax for tracking. with a different indent, i.e. a wrap computed for a different column count, left on screen.
• Line 5 is absent entirely: the gutter goes 4, then 6 through StoreKit 2 and credited by ..., where line 6 starts mid-sentence. Line 5 (a long wrapped line) appears to have been dropped from the repaint.

The first screenshot also shows the user prompt line (❯ go ahead) and the surrounding context rendering fine, so the damage is confined to the region that was being drawn when the resize happened.

Screenshots: (attached below)

What Should Happen?

After a pane resize the TUI should fully repaint the viewport at the new width: every line reflowed for the current column count, no empty rows left behind from the previous layout, and no rows from the previous layout left in place.

Error Messages/Logs

N/A — no error output. Purely a rendering defect.

Steps to Reproduce

1. Open Herdr (0.9.3) on macOS 26.6.2, split the window so Claude Code runs in one pane.
2. Run claude in the pane.
3. Give it a task that produces a long turn with a tool call whose result includes a multi-line file preview (e.g. ask it to write a 500+ line markdown file with Write).
4. While the turn is still running (the Zigzagging… (19m 47s · ↓ 102.2k tokens) spinner is active), drag the Herdr pane divider to change the pane width.
5. Look at the tool result that was on screen during the resize.

Observed: a blank band of several rows after the tool result, plus stale/missing wrapped rows inside the preview. The corruption survives subsequent output being appended below it.

Claude Model

Fable 5.1 (claude-fable-5-1)

Is this a regression?

Not sure

Last Working Version

_No response_

Claude Code Version

2.1.294 (Claude Code)

Platform

Anthropic API

Operating System

macOS 26.6.2 (25G83), Apple Silicon

Terminal/Shell

Herdr 0.9.3 (TERM_PROGRAM=herdr, TERM=xterm-256color), zsh. Node v26.10.0.

Additional Context

Related but distinct:

• #83274 reports stale content after a resize on Windows Terminal; this report is macOS and the stale region also includes a blank band.
• #84247 reports duplicated bands in scrollback after a resize on macOS while the live screen stays correct; here the live viewport itself is wrong.