Summary

Fixes the cursor rewind offset in createIncremental (src/log-update.ts) that causes ghost/duplicate lines when the rendered output has a trailing newline.

Root cause: The "same or grow height" branch uses cursorUp(previousVisible - 1) to rewind to line 0. previousVisible is visibleLineCount() which excludes the trailing empty line from split('\n'). But after writing output ending with '\n', the cursor sits one row past the last visible line. So the rewind falls one row short — line 0 is never reached, all writes land one row too low, and the old line 0 content remains as a ghost line.

Fix: Replace previousVisible - 1 with previousLines.length - 1. This correctly accounts for the cursor position in both cases:

• Trailing '\n' (non-fullscreen): previousLines.length = previousVisible + 1, so length - 1 = previousVisible → rewinds to row 0 ✅
• No trailing '\n' (fullscreen): previousLines.length = previousVisible, so length - 1 = previousVisible - 1 → same as before ✅

The "shrink" branch is unaffected because it already handles trailing newlines via the extraSlot variable.

Changes

One line in src/log-update.ts:

  } else {
-   buffer.push(ansiEscapes.cursorUp(previousVisible - 1));
+   buffer.push(ansiEscapes.cursorUp(previousLines.length - 1));
  }

Test Plan

• Verify with a non-fullscreen app using incrementalRendering: true that state changes update lines in place without ghost lines
• Verify fullscreen apps are unaffected (no behavioral change when output has no trailing newline)
• Verify the "shrink" path still works correctly when output height decreases
• Existing test suite passes (test/log-update.tsx: 40 tests, test/cursor-helpers.tsx: 17 tests)

Related

• Closes Incremental rendering: ghost/duplicate lines when output has trailing newline #909

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