GitHub Copilot
207eddc922
pyte 0.8.2 does not handle CSI S (SU — Scroll Up) or CSI T (SD — Scroll Down). When TERM=xterm-256color, tmux sends CSI n S for bulk scrolling instead of DECSTBM+index. pyte silently ignored these sequences, leaving old content in the screen buffer — visible as ghost content in SVG screenshots. Fix: monkeypatch pyte's CSI dispatch tables to map S→scroll_up and T→scroll_down, and implement both methods on AltScreen with proper scroll-region (DECSTBM) support. Adds 6 tests for SU/SD functionality.
129 lines
5.3 KiB
Markdown
129 lines
5.3 KiB
Markdown
# Ink Partial Clear Fix
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## Problem
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When CLI applications built with [Ink](https://github.com/vadimdemedes/ink) (React for terminals) — such as GitHub Copilot CLI — execute a `/clear` command, the resulting screenshot shows **ghost content**: old conversation lines persist above the fresh prompt.
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The real terminal displays correctly, but the pyte-based screen buffer used for SVG screenshot generation retains the stale content.
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### Example
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Before `/clear`, the screen has 30 lines of conversation. After `/clear`, the user sees only a fresh 6-line prompt, but the screenshot shows:
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```
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Row 0: [old prompt header] ← ghost content
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Row 1: [────────────────]
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Row 2: [❯ hello]
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...
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Row 20: [● old response text] ← ghost content
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Row 21: [/workspace[main]] ← fresh prompt (duplicated)
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Row 22: [────────────────]
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Row 23: [❯ Type @ to mention files]
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Row 24: [────────────────]
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Row 25: [shift+tab cycle mode]
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```
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## Root Cause
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Ink uses a **line-by-line erase** pattern to clear its previous frame before drawing the next one:
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```
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ESC[2K (EL2 — erase entire line)
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ESC[1A (CUU1 — cursor up one row)
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```
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This pair is repeated once per line of the **previous** frame. Ink tracks how many lines it rendered and erases exactly that many before redrawing.
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When `/clear` is issued:
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1. Ink resets its internal "previous output height" counter to 0.
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2. On the next render cycle, Ink erases **0 old lines** (counter is 0), then draws the fresh prompt (~6 lines).
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3. The subsequent render erases 6 lines (the prompt it just drew), redraws — correct from now on.
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In a real terminal, the old content has already **scrolled into the scrollback buffer** and is invisible. But pyte's fixed-size `Screen` keeps all content in the visible buffer. The 6-line erase only clears rows at the bottom, leaving rows 0–24 with orphaned old content.
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### Byte-level example
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Cursor at row 30 after rendering a full conversation:
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```
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Frame N (normal): EL2+CUU1 × 28 (clears rows 30→2) + redraw 28 lines ✓
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/clear resets counter
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Frame N+1 (broken): EL2+CUU1 × 6 (clears rows 30→24) + redraw 6 lines ✗
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Rows 0–23 still contain old content!
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```
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## Fix
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### Primary fix: CSI S (Scroll Up) and CSI T (Scroll Down) support
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The root cause is that **pyte does not implement `CSI S` (SU — Scroll Up) or `CSI T` (SD — Scroll Down)**. When `TERM=xterm-256color` is set, tmux uses `CSI n S` to scroll content up in the outer terminal instead of the DECSTBM + index approach used with simpler TERM types. Without SU support, pyte silently ignores these scroll commands, leaving old content in place.
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The fix monkeypatches pyte's `ByteStream.csi` and `Stream.csi` dispatch tables to map `"S"` → `scroll_up` and `"T"` → `scroll_down`, and adds the corresponding methods to `AltScreen`.
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```python
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# In alt_screen.py — patch pyte's CSI dispatch
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pyte.ByteStream.csi["S"] = "scroll_up"
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pyte.ByteStream.csi["T"] = "scroll_down"
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# AltScreen implements scroll_up() and scroll_down()
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# which shift buffer lines within the scroll region,
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# matching real terminal behaviour.
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```
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### Secondary fix: expand_clear_sequences (best-effort)
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`AltScreen.expand_clear_sequences()` in `alt_screen.py` pre-processes incoming terminal data before it reaches pyte. It detects runs of 3+ `EL2+CUU1` pairs and, if the run doesn't reach row 0, extends it with additional pairs so the erase covers all lines from the cursor position up to the top of the screen.
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```python
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# Before fix: 6 pairs clear rows 30→24, leaving 0–23 dirty
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data = b"\x1b[2K\x1b[1A" * 6
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# After expand_clear_sequences(): 30 pairs clear rows 30→0
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data = screen.expand_clear_sequences(data)
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# Now contains 30 pairs of EL2+CUU1
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```
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Both `DockerExecSession._update_screen()` and `TerminalSession._update_screen()` call this method after C1 normalization and before feeding data to `pyte.ByteStream`.
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### Why this is safe
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- The fix only triggers on runs of **3 or more** `EL2+CUU1` pairs (normal editing uses 1–2 at most).
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- It only **adds** additional erase operations for lines that would already be empty in a real terminal (they scrolled into scrollback).
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- The extra erases are no-ops if the lines are already blank.
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- Short runs (< 3 pairs) and runs that already reach row 0 are left unchanged.
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## Reproducing
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```python
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from webterm.alt_screen import AltScreen
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import pyte
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screen = AltScreen(132, 45)
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stream = pyte.ByteStream(screen)
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# Draw 27 lines of content
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for i in range(27):
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stream.feed(f"Content line {i}\r\n".encode())
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# Ink /clear: only erases 6 lines
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partial_clear = b"\x1b[2K\x1b[1A" * 6 + b"\x1b[2K\x1b[G"
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# Without fix: old content remains on rows 0–20
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# With fix: all rows are cleared
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expanded = screen.expand_clear_sequences(partial_clear)
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stream.feed(expanded)
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# Draw fresh prompt
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stream.feed(b"Fresh prompt\r\n")
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non_empty = [line for line in screen.display if line.strip()]
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assert len(non_empty) == 1 # Only "Fresh prompt"
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```
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## Related
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- `WEBTERM_SCREENSHOT_FORCE_REDRAW` env var — sends SIGWINCH to force app redraw before screenshots, but doesn't fix this issue since Ink still only erases its tracked line count.
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- `docs/tmux-da-response-filtering.md` — another terminal compatibility fix.
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- pyte's known limitations with partial screen clearing are noted in `README.md`.
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