docs(conpty): record the in-box repaint-shape differential (#243) - #246
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Reviewer's GuideThis documentation-only PR records a measured difference between bundled ConPTY v2 and in-box ConPTY v1: v1 reshapes full-screen repaints into relative motion and erasure sequences that can leave stale glyphs after state divergence. It adds a transport-first Sequence diagram for ConPTY repaint-shape triagesequenceDiagram
participant App as FullScreenTUI
participant ConPTY as ConPTY
participant Terminal as TerminalGrid
participant Operator as Investigator
App->>ConPTY: Repaint application screen
alt Bundled v2
ConPTY-->>Terminal: Forward absolute CUP repaint
Terminal->>Terminal: Rewrite every addressed cell
else In-box v1
ConPTY-->>Terminal: Emit CUF, ECH, EL and CR/LF
Terminal->>Terminal: Rewrite only cells conhost marks changed
end
Operator->>ConPTY: Run same scenario with NOCTTY_CONPTY=inbox
Operator->>Terminal: Compare inbox result with bundled result
alt Corruption only with inbox
Operator->>Operator: Classify as transport-shape issue
else Both configurations match
Operator->>Operator: Continue terminal investigation
end
Sequence diagram for captured-stream terminal validationsequenceDiagram
participant Recorder as ConPTYRecorder
participant Stream as CapturedStream
participant VT as libghostty-vt
participant Reference as ReferenceVTModel
Recorder->>Stream: Capture bundled or in-box master output
Stream->>VT: ghostty_terminal_vt_write
VT-->>VT: Update screen and render-state dirty rows
Stream->>Reference: Replay captured bytes
Reference-->>Reference: Update reference screen
VT->>Reference: Compare screen output and dirty-row invariants
alt Byte-identical screens and zero violations
Reference-->>VT: Terminal core not implicated by capture
else Difference detected
Reference-->>VT: Investigate terminal processing
end
Flow diagram for transport-first stray-character triageflowchart TD
Start[Stray leftover characters reported] --> Version[Check active ConPTY source with noctty +version]
Version --> Bundled[Run scenario with bundled ConPTY]
Bundled --> Inbox[Run same scenario with NOCTTY_CONPTY=inbox]
Inbox --> Compare{Results differ?}
Compare -->|Yes| Transport[Investigate in-box repaint shape]
Compare -->|No| Terminal[Continue terminal-state investigation]
Transport --> Caveat[Shape difference alone is not a defect]
Terminal --> Replay[Replay captured streams through libghostty-vt]
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📝 WalkthroughWalkthroughChangesConPTY diagnostic documentation
Priority: ⬇️ Low Estimated code review effort: 2 (Simple) | ~10 minutes Change: Other Suggested reviewers: Merge Risk: 🔵 Low · up to The documentation may misdirect users troubleshooting ConPTY behavior, but the impact is limited to a localized wording fix. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Hey - I've found 3 issues
Prompt for AI Agents
Please address the comments from this code review:
## Individual Comments
### Comment 1
<location path="docs/windows-vt-conformance.md" line_range="149-152" />
<code_context>
+
+To split terminal behaviour from transport behaviour, run the same scenario
+twice on a current build: once as shipped (bundled; confirm with the `ConPTY`
+line in `noctty +version`) and once with `NOCTTY_CONPTY=inbox`. Corruption that
+appears only under `inbox` is transport shape, not terminal state. Releases
+before 1.3.125 have no bundled pair at all, so they always take the in-box path
+and always show the in-box shape.
+
+On the measurement host below, both sources rendered the `herdr` scenario
</code_context>
<issue_to_address>
**issue:** The triage instruction says corruption that appears only with `NOCTTY_CONPTY=inbox` is transport shape and not terminal state, but the two transports emit different VT streams, so a terminal renderer can fail only when processing the in-box stream even when its implementation is otherwise correct. The comparison isolates the transport path as a cause, but does not prove that the terminal state or renderer is uninvolved.
**Triggers:** When a report reproduces only with the in-box transport.
**Suggested fix:** Say that an inbox-only reproduction implicates the transport path and then require stream capture or terminal-core replay before ruling out stream-dependent terminal behavior.
</issue_to_address>
### Comment 2
<location path="docs/windows-vt-conformance.md" line_range="137-142" />
<code_context>
+motion: it walks the row with CUF over cells it believes are already correct,
+erases runs with ECH, and steps between rows with CR/LF.
+
+The practical consequence is that the two sources have different failure modes,
+not just different sequence coverage. CUF moves the cursor without writing, so a
+v1 repaint only rewrites the cells conhost's buffer says changed. If the
+terminal grid and that buffer ever disagree, the stale glyphs sit in the skipped
+gaps and no later repaint clears them; they survive until something forces a
+full redraw. An absolute-CUP repaint from the application rewrites those same
+cells every frame, so the same divergence self-heals. When a user reports stray
+leftover characters in a multiplexer or full-screen TUI, that asymmetry is the
</code_context>
<issue_to_address>
**issue:** The documentation states that skipped gaps survive until a full redraw and that no later repaint clears them, but the same measured v1 output includes ECH and EL, and subsequent application output can explicitly write or erase those cells without a full-screen redraw. The claim turns a possible stale-glyph persistence mode into an unconditional one.
**Triggers:** When a later repaint explicitly touches an affected gap or emits an erase operation.
**Suggested fix:** Describe the gaps as remaining stale until a later write, erase, or full redraw reaches them, rather than saying that no later repaint clears them.
</issue_to_address>
### Comment 3
<location path="docs/windows-capability-matrix.md" line_range="88-91" />
<code_context>
report the active source. On this machine the bundled source delivered measured
Kitty APC and Sixel DCS payloads byte-for-byte; the in-box source dropped both.
-See the generation-specific
+The in-box source also reshapes repaints: it rewrites an application's
+absolute-CUP redraw into relative cursor motion plus ECH, so a terminal grid
+that ever diverges from the conhost buffer keeps stale glyphs in the skipped
+gaps. Releases before 1.3.125 have no bundled pair and always take that path;
+`NOCTTY_CONPTY=inbox` reproduces it on current builds. See the
+generation-specific
[transport catalog](windows-vt-conformance.md#conpty-transport-generations-and-mangling-catalog).
</code_context>
<issue_to_address>
**nitpick:** The matrix presents one `herdr` scenario measured on one in-box conhost as a general rule that the in-box source rewrites absolute-CUP repaints into relative motion plus ECH. Other applications and conhost vintages can produce different repaint shapes, so this wording overstates the evidence and conflicts with the catalog's scenario- and vintage-specific qualification.
**Triggers:** When a reader applies the matrix statement to a different full-screen application or Windows conhost vintage.
**Suggested fix:** Qualify the paragraph as a measured behavior for the documented host/scenario and say that the in-box implementation can reshape repaints rather than asserting this exact CUF/ECH transformation universally.
```suggestion
In the documented `herdr` scenario on this in-box conhost, the source reshaped
the repaint by rewriting the application's absolute-CUP redraw into relative
cursor motion plus ECH. This measured behavior shows that the in-box
implementation can reshape repaints; other applications or conhost vintages
may produce different repaint shapes. A terminal grid that ever diverges from
the conhost buffer keeps stale glyphs in the skipped gaps. Releases before
1.3.125 have no bundled pair and always take that path;
```
</issue_to_address>Sourcery assessment
Approval pending. 2 findings to address first.
Blocking findings: docs/windows-vt-conformance.md:152, docs/windows-vt-conformance.md:142
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Inline comments:
In `@docs/windows-vt-conformance.md`:
- Around line 167-169: Update the resize discussion in the “Resize and reflow”
section to qualify the no-screen-retransmission behavior specifically to the
bundled v2 ConPTY path. Explicitly preserve the distinction that v1 reflows
conhost’s buffer and emits a synthesized repaint into the pipe, while v2
continues with its delta stream without a snapshot.
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The transport catalog only documented sequence stripping (Kitty APC, Sixel DCS), which left the most common in-box failure mode undescribed. ConPTY v1 re-renders its own text buffer, so it also rewrites the shape of a repaint: recording the master side of one scripted 30 s herdr 0.8.2 scenario at 160x45 under both sources, the bundled v2 stream carried 22803 CUP and no CUF or ECH (the application's own absolute-CUP redraw, forwarded verbatim), while the in-box v1 stream carried 6988 CUP plus 15138 CUF, 1933 ECH, 45 EL and 229 LF. That difference changes the failure mode, not just sequence coverage. CUF moves without writing, so a v1 repaint rewrites only the cells conhost's buffer believes changed; any divergence between that buffer and the terminal grid then survives in the skipped gaps until something forces a full redraw, whereas an application's absolute-CUP repaint overwrites the same cells every frame and self-heals. That asymmetry is what a "stray leftover characters" report should rule out first, and the doc now says how: run the scenario twice on a current build, once as shipped and once with NOCTTY_CONPTY=inbox. Both sources rendered the measured herdr scenario correctly on the measurement host and their window captures were pixel-identical, so the shape difference is recorded as a failure-mode asymmetry, not as a defect on this conhost.
Independent review caught that the first draft claimed an absolute-CUP repaint "rewrites those same cells every frame, so the same divergence self-heals". That is false and the capture disproves it: the application's own repaint also skips runs it believes unchanged, e.g. `CSI 4;28H` `Name:` `CSI 4;52H` `Email` `CSI 4;58H` `Ingestion` leaves the gaps between the words untouched. Replaces that mechanism with the two differences the measurements do support. Addressing: every v2 run carries an absolute CUP that re-anchors the cursor, while v1 moves with CUF/CR/LF/ECH whose effect depends on margins, the bottom row, and pending wrap, so one modelling disagreement propagates with nothing to correct it. Granularity: v1 skips single cells inside a word -- the in-box capture contains literal `Modif` `CSI 1C` `ed` and `9/1` `CSI 1C` `/2026` -- so a divergence surfaces as one wrong letter, which an application repaint skipping a whitespace gap cannot produce. Review also questioned whether the bundled capture is really the application's bytes rather than a v2 re-render, so this adds the control that settles it: a child writing a fixed marker sequence came back byte-for-byte under the bundled source (only a `CSI 1t CSI c CSI ?1004h CSI ?9001h` prologue added), and came back under the in-box source as a synthesized top-to-bottom `EL`+CR/LF redraw with overwritten text absent and the child's own CUF replaced by spaces. Adds resize as the plausible origin of a rigid whole-frame offset between the two models, since ConPTY reflows conhost's buffer by its own rules, never retransmits the screen, and continues the delta stream from there.
Review flagged that "corruption that appears only under `inbox` is transport shape, not terminal state" claims more than the experiment shows. The two sources emit different streams, so a terminal bug in exactly the code the v1 shape exercises -- CUF at the right margin, ECH against pending wrap, LF at the bottom row -- would also reproduce only under `inbox`. The step now says an inbox-only reproduction implicates the in-box path rather than the transport, and makes the replay through `libghostty-vt` the deciding move, spelling out what each outcome means: corruption in the replay is a noctty bug with the capture as its regression test; a correct replay leaves either an upstream divergence or the two things a capture cannot hold, live resize/reflow ordering and the GPU renderer. Review also noted the resize sentence contradicted the "Resize and reflow" row in the same document, which records that a v1 resize repaints into the pipe. Replaces the blanket "the screen is not retransmitted" with what was measured here: across three `ResizePseudoConsole` calls, the bundled path emitted no snapshot of its own and the following bytes were the application's own `CSI ?2026h` `CSI 2J` `CSI 1;1H` redraw, while the in-box path emitted its own `CSI ?25l` `CSI H` repaint from home with ECH for the blanks and no ED at all (only the first, shrinking resize also emitted `CSI 8;30;120t`). Overwriting from home without clearing after an independent reflow is the case that can leave cells behind, which is the point the paragraph now makes. Third, the capability-matrix paragraph stated the CUF/ECH transformation as a general property of the in-box source. It is now scoped to the measured `herdr` scenario on this conhost vintage, with "can also reshape repaints" as the general claim, matching the catalog's existing vintage qualification. The closing note that both sources rendered the scenario correctly now says explicitly that this included repeated live window resizes.
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Inline comments:
In `@AGENTS.md`:
- Line 46: In the documented triage guidance, replace the ambiguous “PATH”
reference with “in-box path” when discussing NOCTTY_CONPTY=inbox, while
preserving the existing meaning that an inbox-only reproduction implicates that
transport path.
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Summary
Documents the in-box ConPTY (v1) repaint-shape differential in the transport
catalog, and points "stray leftover characters" triage at it before the terminal.
No code change.
This came out of investigating #243 (herdr renders scrambled on noctty 1.3.124,
Windows 23H2). I could not reproduce the corruption on this host in any
configuration, so this PR does not claim to fix that issue and does not close
it. What the investigation did produce is a measured transport difference the
catalog did not describe, plus the triage recipe that would let the reporter or
the next investigator split transport from terminal in one step.
Root cause of the documentation gap
docs/windows-vt-conformance.mddescribed v1 only as "can drop, reorder, orsynthesize sequences", with a measurement table covering whole-sequence
stripping (Kitty APC, Sixel DCS). That misses what a re-rendering transport
actually does to a full-screen TUI: v1 does not forward the application's
repaint, it emits its own, and its own moves the cursor relatively and skips at
single-cell resolution.
Change
docs/windows-vt-conformance.md: new "Measured repaint-shape differential forfull-screen multiplexers" subsection — sequence counts, the passthrough
control that identifies which stream is whose, the two differences that
actually matter (absolute re-anchoring vs context-dependent relative motion;
whole-run vs single-cell-inside-a-word skip granularity), resize as the
plausible origin of a rigid whole-frame offset, and the
NOCTTY_CONPTY=inboxvs default triage recipe.docs/windows-capability-matrix.md: one-paragraph pointer from the existing"ConPTY transport" section.
AGENTS.md: one Self-Correction Log entry recording the traps this cost (apseudo-console recorder needs
STARTF_USESTDHANDLESwith NULL std handles orthe child bypasses the PTY;
libghostty-vt+ctypessettles terminal-corequestions without a GUI repro; and the wrong mechanism I drafted first).
Verification evidence
Measurement host: Windows
10.0.26200.0, in-boxSystem32\conhost.exeFileVersion
10.0.26100.1, bundledconpty.dllProductVersion1.24.260710001(the pinned redistributable). Subject:herdr0.8.2 with anInk-style agent-CLI mimic in a pane, same scripted 30 s scenario, 160x45,
recorded on the master side of a pseudo console created from
kernel32vs fromthe bundled
conpty.dll.Passthrough control (this is what lets the table be read as "v2 = the
application's bytes"): a child writing
ESC[?1049h MARK_A\r\nMARK_B\r\n ESC[5;10H MARK_C␠␠␠MARK_D ESC[1;1H ESC[38;2;1;2;3m MARK_E ESC[m ESC[7;1H MARK_F ESC[3C MARK_Gcame back byte-for-byte under bundled (plus a
CSI 1t CSI c CSI ?1004h CSI ?9001hprologue and matching epilogue), and cameback under in-box as a synthesized top-to-bottom
EL+CR/LF redraw withMARK_Aabsent (overwritten byMARK_E) and the child'sESC[3Creplaced byliteral spaces.
Resize, at the byte level (added after review): the same scenario recorded
with three
ResizePseudoConsolecalls, output offset noted at each. Bundledemitted no snapshot of its own — the next bytes were the application's own
CSI ?2026h CSI 2J CSI 1;1Hredraw. In-box emitted its ownCSI ?25l CSI Hrepaint from home with ECH for the blanks and no ED at all; only the first,
shrinking resize also emitted
CSI 8;30;120t. Overwrite-without-clear after anindependent reflow is the case that can leave cells behind.
Three live configurations, same herdr scenario (3 panes) plus five window
resizes, ~160x45:
+versionhas noConPTYline, matching thereport; the ZIP ships no
conpty.dll/OpenConsole.exe): renders correctly.main, bundled (+version:ConPTY: bundled ...conpty.dll):renders correctly.
main,NOCTTY_CONPTY=inbox: renders correctly. Captures for(2) and (3) are pixel-identical (
ImageChops.differencebboxNone).A ConPTY-width probe on 1.3.124 also rules out a grid/pty size disagreement: the
child reports
cols=160, and a 160-character ruler ends flush in the lastcolumn with the next line unwrapped.
Terminal core, offline. Both recordings replayed through
ghostty_terminal_vt_writeinghostty-vt.dll— the one built from this branchand the one shipped inside the 1.3.124 portable ZIP. Both versions produce
byte-identical screens to each other and to an independent reference VT model,
at write-chunk sizes 1/2/3/5/7/13/64/997/4096. The render-state dirty invariant
(a row whose cells changed must be marked dirty, else a dirty-row renderer
leaves stale glyphs) held on both versions for the recorded streams, for 1000
generated cases mixing CUP/CUF/CUB/ECH/EL/ED/IL/DL/SU/SD/DECSTBM with narrow,
wide and ambiguous-width text, and for 60 cases plus the real capture replayed
with
ghostty_terminal_resizeapplied mid-stream: 0 violations. herdr 0.9.0through the in-box path behaves the same. That is the basis for recording the
shape difference as a failure-mode asymmetry rather than a defect on this
conhost, and for not asserting a noctty fix for #243.
Review
An independent reviewer re-derived the screenshot's corruption rule from the
pixels (
rendered[r][c] == correct[r+3][c+1], i.e. a frame whose layout sat3 rows up and 1 column right), falsified the renderer hypothesis from the code
(every stage of the dirty pipeline —
src/terminal/render.zig:433-470,generic.zig:2629-2669,cell.zig:222-248,fg_cells_dirty_from— is at leastrow-granular, so it cannot leave single stale cells inside otherwise-correct
rows), and falsified ambiguous-width as a cause (1.3.124's core agrees with
conhost on every non-ASCII codepoint in the stream).
They also rejected the first draft's mechanism. They were right about
"self-heals": an application repaint skips unchanged runs too, so that sentence
was false and is gone. They were wrong that the bundled capture is a v2
re-render; the passthrough control above was run specifically to settle that,
and the bundled stream is verbatim. Both outcomes are now reflected in the text.
AI assistance
Per
AI_POLICY.md: Claude performed the investigation, built the throwawaymeasurement harnesses (pseudo-console recorder, reference VT model,
libghostty-vtctypes driver, window capture), ran the live configurations,obtained and adjudicated the independent review, and drafted this
documentation. Every number quoted above comes from a run on this host; the
conclusions are stated no more strongly than those runs support, and the
non-reproduction is reported as a non-reproduction.
Summary by Sourcery
Document the measured in-box ConPTY repaint-shape differential and provide guidance for isolating transport, terminal-core, and rendering causes of scrambled TUI output.
Enhancements:
Documentation:
Summary by CodeRabbit