fix(tui): ANSI-aware viewport clamp so styled transcript lines don't truncate

cmd/harnesscli/tui/ansi_clamp_regression_test.go

@@ -0,0 +1,53 @@
+package tui
+
+import (
+	"strings"
+	"testing"
+
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/lipgloss"
+)
+
+// TestTUI1409_ModelViewPreservesANSIStyledTranscriptLine is a regression
+// guard for issue #1409 at the assembled tui.Model integration point, not
+// just the standalone viewport package.
+//
+// Background: glamour (via messagebubble.RenderMarkdown) only emits real
+// ANSI escape sequences when os.Stdout is a real terminal — messagebubble's
+// stdoutIsTerminal probe falls back to the escape-free "notty" style
+// otherwise, which is also why forcing lipgloss's global color profile
+// (lipgloss.SetColorProfile(termenv.TrueColor)) does not make glamour emit
+// escapes in this test process (verified experimentally: RenderMarkdown
+// still produced zero ESC bytes for a code-span line with that profile set).
+// That probe lives in the messagebubble package, which is out of scope for
+// this fix, so this test injects a realistic ANSI-styled transcript line
+// directly into the model's viewport (m.vp) — the same line shape
+// messagebubble/glamour produce for a markdown list item with a styled code
+// span under a real terminal — and exercises it through the full,
+// assembled Model.View() (header/separators/viewport/input/status bar),
+// not just viewport.Model.View() in isolation.
+func TestTUI1409_ModelViewPreservesANSIStyledTranscriptLine(t *testing.T) {
+	cfg := DefaultTUIConfig()
+	m := New(cfg)
+	m2, _ := m.Update(tea.WindowSizeMsg{Width: 40, Height: 24})
+	model := m2.(Model)
+
+	// Same shape as the proven issue #1409 example: a glamour list item with
+	// a truecolor-background code span. Visible width 42, rune count 62 —
+	// the naive rune-count clamp this issue fixed would slice mid-escape at
+	// [:40] runes and drop "with an Add function" entirely.
+	styledLine := "  • Created \x1b[48;2;40;40;40m calc.go \x1b[0m with an Add function"
+	model.vp.AppendLine(styledLine)
+
+	view := model.View()
+
+	if !strings.Contains(view, "Add") {
+		t.Errorf("assembled Model.View() dropped visible transcript content past an ANSI escape; view:\n%q", view)
+	}
+
+	for i, l := range strings.Split(view, "\n") {
+		if w := lipgloss.Width(l); w > model.width {
+			t.Errorf("line %d of assembled Model.View() exceeds width: got %d cells, want <= %d (%q)", i, w, model.width, l)
+		}
+	}
+}

cmd/harnesscli/tui/components/viewport/ansi_clamp_test.go

@@ -0,0 +1,44 @@
+package viewport_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/charmbracelet/lipgloss"
+	"go-agent-harness/cmd/harnesscli/tui/components/viewport"
+)
+
+// TestTUI1409_ANSIStyledLineNotTruncatedByRuneCount reproduces issue #1409:
+// a line containing a real ANSI escape sequence (as glamour emits under a
+// real color profile) has more runes than visible cells. The viewport's
+// horizontal clamp must cut by visible width, not rune count, or trailing
+// visible content is lost even though the escape bytes push it past the
+// rune-count budget.
+func TestTUI1409_ANSIStyledLineNotTruncatedByRuneCount(t *testing.T) {
+	// Styled segment "\x1b[48;2;40;40;40m calc.go \x1b[0m" renders as
+	// " calc.go " (9 visible cells) but contributes many more runes than
+	// that via the escape codes. The full line has 42 visible cells and
+	// 62 runes, matching the diagnosis in issue #1409.
+	line := "  • Created \x1b[48;2;40;40;40m calc.go \x1b[0m with an Add function"
+
+	if got := lipgloss.Width(line); got != 42 {
+		t.Fatalf("test fixture invariant broken: want visible width 42, got %d", got)
+	}
+	if got := len([]rune(line)); got != 62 {
+		t.Fatalf("test fixture invariant broken: want rune count 62, got %d", got)
+	}
+
+	vp := viewport.New(40, 5)
+	vp.AppendLine(line)
+	view := vp.View()
+
+	if !strings.Contains(view, "Add") {
+		t.Errorf("visible content within the width-40 budget was dropped by the clamp; view:\n%q", view)
+	}
+
+	for i, l := range strings.Split(view, "\n") {
+		if w := lipgloss.Width(l); w > 40 {
+			t.Errorf("line %d exceeds viewport width: got %d cells, want <= 40 (%q)", i, w, l)
+		}
+	}
+}

cmd/harnesscli/tui/components/viewport/model.go

@@ -5,6 +5,7 @@ import (
 	"strings"
 
 	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/x/ansi"
 )
 
 // Model is the scrollable viewport for conversation content.
@@ -319,10 +320,12 @@ func (m Model) View() string {
 	}
 
 	for _, line := range visible {
-		runes := []rune(line)
-		if len(runes) > m.width {
-			line = string(runes[:m.width])
-		}
+		// Clamp by visible cell width, not rune count: lines can contain
+		// ANSI escape sequences (glamour output under a real color
+		// profile) whose bytes count as runes but occupy no cells.
+		// ansi.Truncate is ANSI-aware and a no-op when the line already
+		// fits within m.width.
+		line = ansi.Truncate(line, m.width, "")
 		sb.WriteString(line)
 		sb.WriteString("\n")
 	}

docs/logs/engineering-log.md

@@ -6256,3 +6256,59 @@ Skipped creating separate issues for Op/EventMsg protocol (already covered by SS
   execute on this darwin worktree and is verified by
   `GOOS=linux go vet ./internal/harness/tools/` for syntax/type correctness
   only — it needs a real Linux CI run to prove behavior.
+
+# 2026-09-06 (Issue #1409 viewport horizontal clamp truncated ANSI-styled transcript lines)
+
+- Symptom: streamed markdown assistant output in the TUI transcript viewport
+  sometimes lost trailing visible text — a bullet item like "Created
+  `calc.go` with an Add function" could render with "with an Add function"
+  (or more) silently missing — but every unit test for the viewport passed.
+- Cause: `viewport.Model.View()`
+  (`cmd/harnesscli/tui/components/viewport/model.go`) clamped each visible
+  line to the viewport width by rune count: `runes := []rune(line); if
+  len(runes) > m.width { line = string(runes[:m.width]) }`. Under a real
+  color profile, glamour (via `messagebubble.RenderMarkdown`) emits ANSI
+  escape sequences — e.g. a truecolor code-span background is ~20 extra
+  bytes — and each escape byte counts as a rune even though it occupies zero
+  terminal cells. A line with 42 visible cells but 62 runes hit the
+  rune-count budget at rune 40, which fell inside the escape sequence itself,
+  so the slice cut mid-escape and dropped everything after it. Under the
+  `ascii`/`notty` glamour style used when `os.Stdout` is not a real terminal
+  (the case for every test process, and for `messagebubble`'s own
+  `stdoutIsTerminal` probe), there are no escape bytes, so rune count equals
+  cell width and the clamp never mis-cut — which is why the existing test
+  suite never caught this.
+- Fix: replaced the rune slice with
+  `github.com/charmbracelet/x/ansi`'s `Truncate(line, m.width, "")`, which is
+  ANSI-aware (it will not cut inside an escape sequence) and is a no-op when
+  the line already fits within `m.width`. `github.com/charmbracelet/x/ansi`
+  was already an indirect dependency (transitively required by the existing
+  charm ecosystem deps); `go mod tidy` after adding the import promoted it —
+  along with two other packages already imported directly elsewhere in the
+  tree but previously mislabeled indirect (`github.com/creack/pty`,
+  `github.com/coder/acp-go-sdk`) — to the direct `require` block in `go.mod`.
+  No dependency versions changed (`go.sum` is byte-for-byte unchanged).
+- Verification: a new `viewport`-package test
+  (`TestTUI1409_ANSIStyledLineNotTruncatedByRuneCount`) feeds the viewport a
+  line with a real ANSI escape sequence (42 visible cells, 62 runes) at
+  width 40 and asserts the trailing marker word survives and no rendered
+  line exceeds width via `lipgloss.Width`; it fails against the pre-fix
+  clamp (marker dropped) and passes after. A second, model-level regression
+  test in `cmd/harnesscli/tui`
+  (`TestTUI1409_ModelViewPreservesANSIStyledTranscriptLine`) drives the same
+  fixture through the fully assembled `tui.Model` (constructed via `New` +
+  a `tea.WindowSizeMsg`, injected via the model's embedded `m.vp`) and
+  asserts the same invariant over the complete `Model.View()` frame
+  (header/separators/viewport/input/status bar), not just the isolated
+  viewport package — confirmed to fail the same way if the fix in
+  `model.go` is reverted. Glamour could not be made to emit real ANSI
+  escapes in this test process even with `lipgloss.SetColorProfile
+  (termenv.TrueColor)` forced (verified experimentally): `messagebubble`'s
+  own `stdoutIsTerminal` probe is a syscall-level check on `os.Stdout`'s fd,
+  independent of lipgloss's global color profile, so it always resolves to
+  the escape-free `notty` style outside a real terminal. That probe lives in
+  `messagebubble`, outside this fix's file scope, so the model-level test
+  injects a realistic pre-rendered ANSI-styled line directly rather than
+  routing it through glamour. `go test ./cmd/harnesscli/tui/... -race`,
+  `go vet ./cmd/harnesscli/tui/...`, and `go build ./cmd/... ./internal/...`
+  are all clean.

go.mod

@@ -26,7 +26,7 @@ require (
 	modernc.org/sqlite v1.33.1
 )
 
-require github.com/creack/pty v1.1.24 // indirect
+require github.com/creack/pty v1.1.24
 
 require (
 	github.com/Microsoft/go-winio v0.4.21 // indirect
@@ -37,15 +37,15 @@ require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/charmbracelet/colorprofile v0.4.1 // indirect
-	github.com/charmbracelet/x/ansi v0.11.6 // indirect
+	github.com/charmbracelet/x/ansi v0.11.6
 	github.com/charmbracelet/x/cellbuf v0.0.15 // indirect
 	github.com/charmbracelet/x/exp/golden v0.0.0-20241011142426-46044092ad91 // indirect
 	github.com/charmbracelet/x/exp/slice v0.0.0-20250327172914-2fdc97757edf // indirect
 	github.com/charmbracelet/x/term v0.2.2 // indirect
 	github.com/clipperhouse/displaywidth v0.9.0 // indirect
 	github.com/clipperhouse/stringish v0.1.1 // indirect
 	github.com/clipperhouse/uax29/v2 v2.5.0 // indirect
-	github.com/coder/acp-go-sdk v0.13.5 // indirect
+	github.com/coder/acp-go-sdk v0.13.5
 	github.com/containerd/errdefs v1.0.0 // indirect
 	github.com/containerd/errdefs/pkg v0.3.0 // indirect
 	github.com/containerd/log v0.1.0 // indirect