fix(tui): stabilize full-width layout and sparkline rendering

6 files changed, 144 insertions, 105 deletions

diff --git a/internal/tui/dashboard/histogram.go b/internal/tui/dashboard/histogram.go
index 7613230..3511dfb 100644
--- a/internal/tui/dashboard/histogram.go
+++ b/internal/tui/dashboard/histogram.go
@@ -14,9 +14,10 @@ func renderLatencyTab(snap *statsengine.Snapshot, width, height int) string {
 		return common.PanelStyle.Render("Latency: waiting for stats...")
 	}
 
+	panelW := panelWidth(width)
 	panelInner := panelInnerWidth(width)
 	hist := renderHistogram(snap.LatencyHistogram, "Latency Histogram", width, height)
-	spark := common.PanelStyle.Width(panelInner).Render(
+	spark := common.PanelStyle.Width(panelW).Render(
 		renderOverviewSparkline("Latency sparkline:", snap.LatencySeriesNs(), panelInner),
 	)
 	return strings.Join([]string{hist, spark}, "\n")
@@ -27,9 +28,10 @@ func renderGapsTab(snap *statsengine.Snapshot, width, height int) string {
 		return common.PanelStyle.Render("Gaps: waiting for stats...")
 	}
 
+	panelW := panelWidth(width)
 	panelInner := panelInnerWidth(width)
 	hist := renderHistogram(snap.GapHistogram, "Gap Histogram", width, height)
-	spark := common.PanelStyle.Width(panelInner).Render(
+	spark := common.PanelStyle.Width(panelW).Render(
 		renderOverviewSparkline("Gap sparkline:", snap.GapSeriesNs(), panelInner),
 	)
 	return strings.Join([]string{hist, spark}, "\n")
@@ -53,6 +55,7 @@ func renderHistogram(hist statsengine.HistogramSnapshot, title string, width, he
 	if width <= 0 {
 		width = 80
 	}
+	panelW := panelWidth(width)
 	panelInner := panelInnerWidth(width)
 
 	if height > 0 {
@@ -93,7 +96,7 @@ func renderHistogram(hist statsengine.HistogramSnapshot, title string, width, he
 	}
 	lines = append(lines, "Scale: █▓▒░")
 
-	return common.PanelStyle.Width(panelInner).Render(strings.Join(lines, "\n"))
+	return common.PanelStyle.Width(panelW).Render(strings.Join(lines, "\n"))
 }
 
 func renderHistogramBar(count, maxCount uint64, width int) string {
diff --git a/internal/tui/dashboard/layout.go b/internal/tui/dashboard/layout.go
index 0035a9d..75cbafb 100644
--- a/internal/tui/dashboard/layout.go
+++ b/internal/tui/dashboard/layout.go
@@ -4,7 +4,3 @@ const panelHorizontalChrome = 4
 
 // Keep a small guard so sparkline rows never soft-wrap in panel cells.
 const sparklineSafetyMargin = 3
-
-// Stats engine currently provides 120 time-series slots; cap rendering width
-// so wide terminals don't introduce wrap/placement artifacts.
-const sparklineMaxWidth = 120
diff --git a/internal/tui/dashboard/overview.go b/internal/tui/dashboard/overview.go
index 3ddeaf6..866c5bb 100644
--- a/internal/tui/dashboard/overview.go
+++ b/internal/tui/dashboard/overview.go
@@ -33,6 +33,7 @@ func renderOverview(snap *statsengine.Snapshot, width, height int) string {
 		trendWithArrow(snap.ThroughputTrend),
 	)
 
+	panelW := panelWidth(width)
 	panelInner := panelInnerWidth(width)
 	labelWidth := maxLabelWidth("Latency:", "Gap:", "Throughput:")
 	latencySpark := renderOverviewSparklineAligned("Latency:", snap.LatencySeriesNs(), panelInner, labelWidth)
@@ -44,8 +45,8 @@ func renderOverview(snap *statsengine.Snapshot, width, height int) string {
 	latencyHist := "Latency buckets: " + summarizeHistogramBrief(snap.LatencyHistogram)
 	gapHist := "Gap buckets: " + summarizeHistogramBrief(snap.GapHistogram)
 
-	panel := common.PanelStyle.Width(panelInner)
-	sparkPanel := panel.Render(strings.Join([]string{latencySpark, "", gapSpark, "", throughputSpark}, "\n"))
+	panel := common.PanelStyle.Width(panelW)
+	sparkPanel := panel.Render(strings.Join([]string{latencySpark, gapSpark, throughputSpark}, "\n"))
 	topPanel := panel.Render(strings.Join([]string{topSyscalls, topFiles, topProcesses}, "\n"))
 	histPanel := panel.Render(strings.Join([]string{latencyHist, gapHist}, "\n"))
 
@@ -73,7 +74,7 @@ func renderSyscallBox(snap *statsengine.Snapshot, width int) string {
 		snap.SyscallRatePerSec,
 		generatedAt,
 	)
-	return common.PanelStyle.Width(summaryBoxInnerWidth(width)).Height(5).Render(content)
+	return common.PanelStyle.Width(width).Height(5).Render(content)
 }
 
 func renderBytesBox(snap *statsengine.Snapshot, width int) string {
@@ -83,7 +84,7 @@ func renderBytesBox(snap *statsengine.Snapshot, width int) string {
 		formatBytes(snap.WriteBytesPerSec),
 		formatBytes(float64(snap.TotalBytes)),
 	)
-	return common.PanelStyle.Width(summaryBoxInnerWidth(width)).Height(5).Render(content)
+	return common.PanelStyle.Width(width).Height(5).Render(content)
 }
 
 func renderErrorBox(snap *statsengine.Snapshot, width int) string {
@@ -99,7 +100,7 @@ func renderErrorBox(snap *statsengine.Snapshot, width int) string {
 		snap.LatencyMeanNs,
 		snap.GapMeanNs,
 	)
-	return common.PanelStyle.Width(summaryBoxInnerWidth(width)).Height(5).Render(content)
+	return common.PanelStyle.Width(width).Height(5).Render(content)
 }
 
 func trendWithArrow(trend statsengine.Trend) string {
@@ -212,19 +213,8 @@ func summaryBoxWidth(width int) int {
 	return w
 }
 
-func summaryBoxInnerWidth(width int) int {
-	inner := width - panelHorizontalChrome
-	if inner < 14 {
-		return 14
-	}
-	return inner
-}
-
 func renderOverviewSparkline(label string, data []float64, panelInner int) string {
 	w := panelInner - utf8.RuneCountInString(label) - 1 - sparklineSafetyMargin
-	if w > sparklineMaxWidth {
-		w = sparklineMaxWidth
-	}
 	if w < 8 {
 		w = 8
 	}
@@ -234,9 +224,6 @@ func renderOverviewSparkline(label string, data []float64, panelInner int) strin
 func renderOverviewSparklineAligned(label string, data []float64, panelInner int, labelWidth int) string {
 	paddedLabel := padLabelRight(label, labelWidth)
 	w := panelInner - labelWidth - 1 - sparklineSafetyMargin
-	if w > sparklineMaxWidth {
-		w = sparklineMaxWidth
-	}
 	if w < 8 {
 		w = 8
 	}
@@ -262,13 +249,20 @@ func padLabelRight(label string, width int) string {
 	return label + strings.Repeat(" ", pad)
 }
 
-func panelInnerWidth(width int) int {
+func panelWidth(width int) int {
 	if width <= 0 {
 		width = 80
 	}
-	inner := width - panelHorizontalChrome
-	if inner < 20 {
+	if width < 20 {
 		return 20
 	}
+	return width
+}
+
+func panelInnerWidth(width int) int {
+	inner := panelWidth(width) - panelHorizontalChrome
+	if inner < 16 {
+		return 16
+	}
 	return inner
 }
diff --git a/internal/tui/dashboard/overview_test.go b/internal/tui/dashboard/overview_test.go
index 7de411c..6ac3704 100644
--- a/internal/tui/dashboard/overview_test.go
+++ b/internal/tui/dashboard/overview_test.go
@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"ior/internal/statsengine"
+	common "ior/internal/tui/common"
 
 	"charm.land/lipgloss/v2"
 )
@@ -121,23 +122,22 @@ func TestRenderOverviewDoesNotOverflowWidth(t *testing.T) {
 func TestRenderOverviewSparklineHasSafetyMargin(t *testing.T) {
 	const panelInner = 80
 	out := renderOverviewSparkline("Latency:", []float64{1, 2, 3, 4, 5}, panelInner)
-	lines := strings.Split(out, "\n")
-	if len(lines) != 2 {
-		t.Fatalf("expected 2-line sparkline, got %q", out)
+	if strings.Contains(out, "\n") {
+		t.Fatalf("expected single-line sparkline, got %q", out)
 	}
-	if got, max := lipgloss.Width(lines[0]), panelInner-sparklineSafetyMargin; got > max {
+	if got, max := lipgloss.Width(out), panelInner-sparklineSafetyMargin; got > max {
 		t.Fatalf("expected sparkline width <= %d with safety margin, got %d", max, got)
 	}
 }
 
-func TestRenderOverviewSparklineCapsWidth(t *testing.T) {
+func TestRenderOverviewSparklineUsesAvailableWidth(t *testing.T) {
 	out := renderOverviewSparkline("Latency:", make([]float64, 120), 400)
-	lines := strings.Split(out, "\n")
-	if len(lines) != 2 {
-		t.Fatalf("expected 2-line sparkline, got %q", out)
+	if strings.Contains(out, "\n") {
+		t.Fatalf("expected single-line sparkline, got %q", out)
 	}
-	if got := lipgloss.Width(lines[0]) - len("Latency: "); got > sparklineMaxWidth {
-		t.Fatalf("expected capped sparkline width <= %d, got %d", sparklineMaxWidth, got)
+	want := 400 - len("Latency:") - 1 - sparklineSafetyMargin
+	if got := lipgloss.Width(out) - len("Latency: "); got != want {
+		t.Fatalf("expected sparkline width %d, got %d", want, got)
 	}
 }
 
@@ -164,3 +164,14 @@ func TestRenderOverviewSparklineAlignedUsesSameSparkStartColumn(t *testing.T) {
 		t.Fatalf("unexpected throughput prefix: %q", thrTop)
 	}
 }
+
+func TestRenderOverviewSparklineAlignedFitsSinglePanelRow(t *testing.T) {
+	panelW := panelWidth(220)
+	panelInner := panelInnerWidth(220)
+	labelWidth := maxLabelWidth("Latency:", "Gap:", "Throughput:")
+	line := renderOverviewSparklineAligned("Latency:", []float64{0, 10, 5, 10, 0}, panelInner, labelWidth)
+	rendered := common.PanelStyle.Width(panelW).Render(line)
+	if got := len(strings.Split(rendered, "\n")); got != 3 {
+		t.Fatalf("expected sparkline to fit one panel row (3 total lines with border), got %d lines", got)
+	}
+}
diff --git a/internal/tui/dashboard/sparkline.go b/internal/tui/dashboard/sparkline.go
index 2ce8c90..ab78cce 100644
--- a/internal/tui/dashboard/sparkline.go
+++ b/internal/tui/dashboard/sparkline.go
@@ -1,9 +1,8 @@
 package dashboard
 
 import "math"
-import "strings"
 
-var sparkRowChars = []rune(" ▁▂▃▄▅▆▇█")
+var sparkChars = []rune("▁▂▃▄▅▆▇█")
 
 func renderSparkline(data []float64, width int) string {
 	if len(data) == 0 || width <= 0 {
@@ -11,23 +10,15 @@ func renderSparkline(data []float64, width int) string {
 	}
 
 	samples := sampleForWidth(data, width)
-	leftPad := 0
-	if len(samples) < width {
-		leftPad = width - len(samples)
-	}
 	min, max := minMax(samples)
 	if min == max {
-		top := repeatRune(' ', width)
-		bottom := repeatRune(' ', leftPad) + repeatRune('█', len(samples))
-		return top + "\n" + bottom
+		if min == 0 {
+			return repeatRune(' ', width)
+		}
+		return repeatRune('▁', width)
 	}
 
-	top := make([]rune, width)
-	bottom := make([]rune, width)
-	for i := 0; i < leftPad; i++ {
-		top[i] = ' '
-		bottom[i] = ' '
-	}
+	row := make([]rune, width)
 	scale := 16.0
 	denom := max - min
 	for i, value := range samples {
@@ -39,20 +30,17 @@ func renderSparkline(data []float64, width int) string {
 			level = 16
 		}
 
-		topLevel := level - 8
-		if topLevel < 0 {
-			topLevel = 0
+		// Collapse the previous two-row 0..16 scale to a single-row 0..7 scale.
+		oneRow := level / 2
+		if oneRow < 0 {
+			oneRow = 0
 		}
-		bottomLevel := level
-		if bottomLevel > 8 {
-			bottomLevel = 8
+		if oneRow > 7 {
+			oneRow = 7
 		}
-
-		col := leftPad + i
-		top[col] = sparkRowChars[topLevel]
-		bottom[col] = sparkRowChars[bottomLevel]
+		row[i] = sparkChars[oneRow]
 	}
-	return string(top) + "\n" + string(bottom)
+	return string(row)
 }
 
 func renderLabeledSparkline(label string, data []float64, width int) string {
@@ -60,20 +48,47 @@ func renderLabeledSparkline(label string, data []float64, width int) string {
 	if spark == "" {
 		return label
 	}
-	lines := strings.Split(spark, "\n")
-	if len(lines) == 1 {
-		return label + " " + lines[0]
-	}
-	pad := repeatRune(' ', len([]rune(label))+1)
-	return label + " " + lines[0] + "\n" + pad + lines[1]
+	return label + " " + spark
 }
 
 func sampleForWidth(data []float64, width int) []float64 {
-	if width >= len(data) {
+	if width <= 0 || len(data) == 0 {
+		return nil
+	}
+
+	if width < len(data) {
+		start := len(data) - width
+		return append([]float64(nil), data[start:]...)
+	}
+
+	if width == len(data) {
 		return append([]float64(nil), data...)
 	}
-	start := len(data) - width
-	return append([]float64(nil), data[start:]...)
+
+	if len(data) == 1 {
+		out := make([]float64, width)
+		for i := range out {
+			out[i] = data[0]
+		}
+		return out
+	}
+
+	out := make([]float64, width)
+	srcLast := len(data) - 1
+	dstLast := width - 1
+	for i := 0; i < width; i++ {
+		// Nearest-neighbor upsampling preserves the original series shape
+		// without introducing interpolated spikes between samples.
+		srcIdx := int(math.Round(float64(i) * float64(srcLast) / float64(dstLast)))
+		if srcIdx < 0 {
+			srcIdx = 0
+		}
+		if srcIdx > srcLast {
+			srcIdx = srcLast
+		}
+		out[i] = data[srcIdx]
+	}
+	return out
 }
 
 func minMax(values []float64) (float64, float64) {
diff --git a/internal/tui/dashboard/sparkline_test.go b/internal/tui/dashboard/sparkline_test.go
index d7acd33..6f549d1 100644
--- a/internal/tui/dashboard/sparkline_test.go
+++ b/internal/tui/dashboard/sparkline_test.go
@@ -16,37 +16,52 @@ func TestRenderSparklineEmptyOrInvalidWidth(t *testing.T) {
 
 func TestRenderSparklineSingleValue(t *testing.T) {
 	got := renderSparkline([]float64{10}, 8)
-	if got != "        \n       █" {
-		t.Fatalf("expected two-line constant sparkline, got %q", got)
+	if got != "▁▁▁▁▁▁▁▁" {
+		t.Fatalf("expected single-line constant sparkline, got %q", got)
 	}
 }
 
 func TestRenderSparklineAllEqualValues(t *testing.T) {
 	got := renderSparkline([]float64{5, 5, 5, 5}, 4)
-	if got != "    \n████" {
-		t.Fatalf("expected two-line flat sparkline, got %q", got)
+	if got != "▁▁▁▁" {
+		t.Fatalf("expected single-line flat sparkline, got %q", got)
 	}
 }
 
-func TestRenderSparklineRightAlignsShortHistory(t *testing.T) {
+func TestRenderSparklineAllZeroValuesRendersBlank(t *testing.T) {
+	got := renderSparkline([]float64{0, 0, 0}, 5)
+	if got != "     " {
+		t.Fatalf("expected blank sparkline for all-zero series, got %q", got)
+	}
+}
+
+func TestRenderSparklineLeftAlignsShortHistory(t *testing.T) {
 	got := renderSparkline([]float64{1, 2, 3}, 6)
-	lines := strings.Split(got, "\n")
-	if len(lines) != 2 {
-		t.Fatalf("expected 2 lines, got %q", got)
+	first := strings.IndexFunc(got, func(r rune) bool { return r != ' ' })
+	last := strings.LastIndexFunc(got, func(r rune) bool { return r != ' ' })
+	if first < 0 || last < 0 {
+		t.Fatalf("expected visible sparkline cells, got %q", got)
+	}
+	if strings.HasPrefix(got, "   ") {
+		t.Fatalf("expected sparkline not to use old right-aligned padding, got %q", got)
 	}
-	if !strings.HasPrefix(lines[1], "   ") {
-		t.Fatalf("expected left padding for short history, got %q", lines[1])
+}
+
+func TestRenderSparklineUsesRightmostColumn(t *testing.T) {
+	got := renderSparkline([]float64{1, 3, 2, 5}, 20)
+	row := []rune(got)
+	if len(row) != 20 {
+		t.Fatalf("expected 20 columns, got %d", len(row))
+	}
+	if row[19] == ' ' {
+		t.Fatalf("expected rightmost column to contain sparkline data, got %q", got)
 	}
 }
 
 func TestRenderSparklineRespectsWidthTruncation(t *testing.T) {
 	got := renderSparkline([]float64{1, 2, 3, 4, 5, 6, 7, 8}, 4)
-	lines := strings.Split(got, "\n")
-	if len(lines) != 2 {
-		t.Fatalf("expected 2 lines, got %q", got)
-	}
-	if len([]rune(lines[0])) != 4 || len([]rune(lines[1])) != 4 {
-		t.Fatalf("expected 4 runes per line, got %q", got)
+	if len([]rune(got)) != 4 {
+		t.Fatalf("expected 4 runes, got %q", got)
 	}
 }
 
@@ -63,27 +78,32 @@ func TestSampleForWidthUsesRecentTail(t *testing.T) {
 	}
 }
 
+func TestSampleForWidthUpsamplesToFullWidth(t *testing.T) {
+	got := sampleForWidth([]float64{10, 20, 30}, 7)
+	if len(got) != 7 {
+		t.Fatalf("expected 7 samples, got %d", len(got))
+	}
+	if got[0] != 10 {
+		t.Fatalf("expected first sample to preserve series start, got %v", got[0])
+	}
+	if got[len(got)-1] != 30 {
+		t.Fatalf("expected last sample to preserve series end, got %v", got[len(got)-1])
+	}
+}
+
 func TestRenderSparklineSpansLowToHigh(t *testing.T) {
 	got := renderSparkline([]float64{0, 10}, 2)
-	lines := strings.Split(got, "\n")
-	if len(lines) != 2 {
-		t.Fatalf("expected 2 lines, got %q", got)
-	}
-	if !strings.Contains(got, "█") {
-		t.Fatalf("expected high bar, got %q", got)
+	if got != "▁█" {
+		t.Fatalf("expected low-to-high one-row sparkline, got %q", got)
 	}
 }
 
-func TestRenderLabeledSparklineAlignsSecondRow(t *testing.T) {
+func TestRenderLabeledSparklineSingleLine(t *testing.T) {
 	got := renderLabeledSparkline("Latency:", []float64{0, 10}, 2)
-	lines := strings.Split(got, "\n")
-	if len(lines) != 2 {
-		t.Fatalf("expected 2 lines, got %q", got)
-	}
-	if !strings.HasPrefix(lines[0], "Latency: ") {
-		t.Fatalf("expected label prefix on first row, got %q", lines[0])
+	if strings.Contains(got, "\n") {
+		t.Fatalf("expected single-line labeled sparkline, got %q", got)
 	}
-	if !strings.HasPrefix(lines[1], "         ") {
-		t.Fatalf("expected padding on second row to align sparkline, got %q", lines[1])
+	if !strings.HasPrefix(got, "Latency: ") {
+		t.Fatalf("expected label prefix, got %q", got)
 	}
 }