Bug Description

After resizing the terminal window (shrink then enlarge, or vice versa), the Hermes TUI enters an infinite render loop:

• The status bar line duplicates/scrolls repeatedly
• Screen flickers as fullReset is called every frame
• The only escape is Ctrl+C to kill the process

Reproduction Steps

1. Start hermes in macOS Terminal (or iTerm2)
2. Wait for the welcome banner + prompt to appear
3. Shrink the terminal window (drag edge to reduce height)
4. Enlarge it back to original size
5. Observe: status bar lines start stacking, screen flickers, prompt becomes unusable

Environment

• Hermes version: v0.12.0 (2026.4.30), upstream 5d3be89
• OS: macOS 15.x on MacBook Air
• Terminal: macOS Terminal.app and iTerm2 (both reproduce)
• Shell: zsh 5.9

Root Cause Analysis

The root cause is in ui-tui/packages/hermes-ink/src/ink/log-update.ts around line 230:

if (prev.screen.height >= prev.viewport.height && prev.screen.height > 0 && cursorAtBottom && !isGrowing) {
    // scrollback-diff check
    if (scrollbackChangeY >= 0) {
        return fullResetSequence_CAUSES_FLICKER(next, "offscreen", stylePool, ...)
    }
}

This block detects changes in scrollback rows (off-screen rows above the viewport) and triggers fullResetSequence_CAUSES_FLICKER. The loop:

1. Resize triggers fullReset (line ~150, viewport.height < prev.viewport.height) — expected and correct.
2. fullReset clears the entire terminal and repaints. After repaint, cursor-restore LF may push rows into scrollback differently than before.
3. Next frame: scrollback-diff block detects that scrollback rows changed (because fullReset repositioned the cursor), triggers another fullReset.
4. Infinite loop: each fullReset causes scrollback changes that trigger the next fullReset.

The loop is especially triggered when:

• A spinner/timer/progress element updates in a scrollback row
• The status bar content changes between frames (e.g. timer display ⏲ Ns)
• Any dynamic content exists in what the renderer considers scrollback

The scrollback-diff block at line ~230 has NO altScreen guard, so it fires in main-screen mode too, where scrollback rows are truly off-screen and invisible.

Suggested Fix

Option A: Gate the scrollback-diff fullReset behind altScreen mode. In main-screen mode, scrollback row changes are invisible to the user (above the visible viewport), so incremental diff of visible rows is sufficient.

Option B: After a resize-triggered fullReset, add a one-frame cooldown that suppresses the scrollback-diff check, preventing the immediate re-trigger.

Additional Context

This affects daily usability — any terminal resize during a session makes the TUI unusable until restart. Confirmed with debug logging showing repeated fullReset(offscreen) calls triggered by scrollback-diff matching spinner/status-bar row changes.

Debug log evidence:

• Every frame: [SCROLL-DBG] resize fullReset followed by fullReset(offscreen) on next frame
• The offscreen trigger shows scrollback rows differing due to previous frame fullReset repositioning content
• Loop continues until process is killed

Workaround

No known workaround other than avoiding terminal resize while Hermes TUI is running.