fix: stderr panel content now expands when focused

The stderr viewport's scroll offset was calculated using the small (20%)
height, so GotoBottom() positioned the view incorrectly when the panel
expanded to 80%. Viewport dimensions are now updated on every panel switch.

Author: juanibiapina

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- **TUI: Fix stderr panel content not expanding when focused**: When selecting the stderr panel (via `5` or tab), the panel border grew to 80% of the screen but the log content stayed at the original 20% size with empty space below. The viewport's scroll offset was calculated using the small height, so `GotoBottom()` positioned the view incorrectly for the larger panel. Viewport dimensions are now updated whenever panel focus changes.
+
 ## [3.2.0] - 2026-02-07
 
 ### Changed

internal/tui/tui.go

@@ -401,24 +401,22 @@ func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		logWidth := m.width - m.jobPanelWidth()
 		totalLogHeight := m.height - 2 // header + status bar
 
-		// Stderr gets 20% of height, stdout gets 80%
-		stderrHeight := totalLogHeight * 20 / 100
-		if stderrHeight < 4 {
-			stderrHeight = 4
-		}
-		stdoutHeight := totalLogHeight - stderrHeight
-
 		// Store log panel width for line wrapping
 		m.logPanelWidth = logWidth - 4
 
-		m.stdoutView = viewport.New(logWidth-4, stdoutHeight-3)
-		m.stderrView = viewport.New(logWidth-4, stderrHeight-3)
+		// Create viewports with initial dimensions (updateLogViewportSizes
+		// will set the correct heights based on activePanel)
+		m.stdoutView = viewport.New(logWidth-4, 0)
+		m.stderrView = viewport.New(logWidth-4, 0)
 		m.jobListView = viewport.New(m.jobPanelWidth()-4, totalLogHeight-3)
 
 		// Enable horizontal scrolling on log viewports
 		m.stdoutView.SetHorizontalStep(4)
 		m.stderrView.SetHorizontalStep(4)
 
+		// Set correct viewport sizes based on active panel
+		m.updateLogViewportSizes()
+
 		// Calculate visible rows for scrollable panels (matches renderPanels layout)
 		totalH := m.height - 1 // height - status bar
 		if totalH < 8 {
@@ -760,22 +758,27 @@ func (m Model) updateMain(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 
 	case "1":
 		m.activePanel = panelJobs
+		m.updateLogViewportSizes()
 		telemetry.TUIActionExecute("switch_panel")
 
 	case "2":
 		m.activePanel = panelPorts
+		m.updateLogViewportSizes()
 		telemetry.TUIActionExecute("switch_panel")
 
 	case "3":
 		m.activePanel = panelRuns
+		m.updateLogViewportSizes()
 		telemetry.TUIActionExecute("switch_panel")
 
 	case "4":
 		m.activePanel = panelStdout
+		m.updateLogViewportSizes()
 		telemetry.TUIActionExecute("switch_panel")
 
 	case "5":
 		m.activePanel = panelStderr
+		m.updateLogViewportSizes()
 		telemetry.TUIActionExecute("switch_panel")
 
 	case "tab":
@@ -791,6 +794,7 @@ func (m Model) updateMain(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		case panelStderr:
 			m.activePanel = panelJobs
 		}
+		m.updateLogViewportSizes()
 		telemetry.TUIActionExecute("switch_panel")
 
 	case "shift+tab":
@@ -806,6 +810,7 @@ func (m Model) updateMain(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		case panelStderr:
 			m.activePanel = panelStdout
 		}
+		m.updateLogViewportSizes()
 		telemetry.TUIActionExecute("switch_panel")
 
 	case "?":
@@ -1369,6 +1374,45 @@ func (m Model) jobPanelWidth() int {
 	return w
 }
 
+// logPanelHeights returns the stdout and stderr panel heights based on the
+// current active panel. Stderr expands to 80% when focused, otherwise 20%.
+func (m Model) logPanelHeights() (stdoutH, stderrH int) {
+	totalH := m.height - 1 // height - status bar
+	if totalH < 8 {
+		totalH = 8
+	}
+
+	if m.activePanel == panelStderr {
+		stderrH = totalH * 80 / 100
+	} else {
+		stderrH = totalH * 20 / 100
+	}
+	if stderrH < 4 {
+		stderrH = 4
+	}
+	stdoutH = totalH - stderrH
+	return stdoutH, stderrH
+}
+
+// updateLogViewportSizes recalculates the stdout/stderr viewport dimensions
+// based on the current active panel and window size. This must be called
+// whenever activePanel or window size changes so that GotoBottom and scroll
+// operations use the correct height.
+func (m *Model) updateLogViewportSizes() {
+	rightPanelW := m.width - m.jobPanelWidth()
+	stdoutH, stderrH := m.logPanelHeights()
+
+	m.stdoutView.Width = rightPanelW - 4
+	m.stdoutView.Height = stdoutH - 3
+	m.stderrView.Width = rightPanelW - 4
+	m.stderrView.Height = stderrH - 3
+
+	if m.followLogs {
+		m.stdoutView.GotoBottom()
+		m.stderrView.GotoBottom()
+	}
+}
+
 // View renders the UI
 func (m Model) View() string {
 	if !m.ready {

internal/tui/tui_test.go

@@ -0,0 +1,177 @@
+package tui
+
+import (
+	"testing"
+)
+
+func TestLogPanelHeights_DefaultPanel(t *testing.T) {
+	// When stderr is not focused, it should get 20% and stdout 80%
+	m := Model{
+		height:      101, // totalH = 100
+		width:       200,
+		activePanel: panelJobs,
+	}
+
+	stdoutH, stderrH := m.logPanelHeights()
+
+	if stderrH != 20 {
+		t.Errorf("stderrH = %d, want 20 (20%% of 100)", stderrH)
+	}
+	if stdoutH != 80 {
+		t.Errorf("stdoutH = %d, want 80 (100 - 20)", stdoutH)
+	}
+}
+
+func TestLogPanelHeights_StderrFocused(t *testing.T) {
+	// When stderr is focused, it should get 80% and stdout 20%
+	m := Model{
+		height:      101, // totalH = 100
+		width:       200,
+		activePanel: panelStderr,
+	}
+
+	stdoutH, stderrH := m.logPanelHeights()
+
+	if stderrH != 80 {
+		t.Errorf("stderrH = %d, want 80 (80%% of 100)", stderrH)
+	}
+	if stdoutH != 20 {
+		t.Errorf("stdoutH = %d, want 20 (100 - 80)", stdoutH)
+	}
+}
+
+func TestLogPanelHeights_StdoutFocused(t *testing.T) {
+	// Stdout focused should behave like default (stderr stays small)
+	m := Model{
+		height:      101,
+		width:       200,
+		activePanel: panelStdout,
+	}
+
+	stdoutH, stderrH := m.logPanelHeights()
+
+	if stderrH != 20 {
+		t.Errorf("stderrH = %d, want 20", stderrH)
+	}
+	if stdoutH != 80 {
+		t.Errorf("stdoutH = %d, want 80", stdoutH)
+	}
+}
+
+func TestLogPanelHeights_MinimumStderrHeight(t *testing.T) {
+	// With very small terminal, stderr should be at least 4
+	m := Model{
+		height:      10, // totalH = 9
+		width:       200,
+		activePanel: panelJobs,
+	}
+
+	stdoutH, stderrH := m.logPanelHeights()
+
+	// 9 * 20 / 100 = 1, which is less than 4
+	if stderrH < 4 {
+		t.Errorf("stderrH = %d, want >= 4 (minimum)", stderrH)
+	}
+	if stdoutH+stderrH != 9 {
+		t.Errorf("stdoutH(%d) + stderrH(%d) = %d, want 9 (totalH)", stdoutH, stderrH, stdoutH+stderrH)
+	}
+}
+
+func TestLogPanelHeights_MinimumTotalHeight(t *testing.T) {
+	// With tiny terminal, totalH should be clamped to 8
+	m := Model{
+		height:      3, // totalH = 2, clamped to 8
+		width:       200,
+		activePanel: panelJobs,
+	}
+
+	stdoutH, stderrH := m.logPanelHeights()
+
+	// totalH clamped to 8, stderrH = 8 * 20 / 100 = 1, clamped to 4
+	if stderrH < 4 {
+		t.Errorf("stderrH = %d, want >= 4 (minimum)", stderrH)
+	}
+	if stdoutH+stderrH != 8 {
+		t.Errorf("stdoutH(%d) + stderrH(%d) = %d, want 8 (clamped totalH)", stdoutH, stderrH, stdoutH+stderrH)
+	}
+}
+
+func TestLogPanelHeights_SumEqualsTotal(t *testing.T) {
+	// Heights should always sum to totalH for all panels
+	panels := []panel{panelJobs, panelPorts, panelRuns, panelStdout, panelStderr}
+
+	for _, p := range panels {
+		m := Model{
+			height:      51, // totalH = 50
+			width:       200,
+			activePanel: p,
+		}
+
+		stdoutH, stderrH := m.logPanelHeights()
+		totalH := 50
+
+		if stdoutH+stderrH != totalH {
+			t.Errorf("panel %d: stdoutH(%d) + stderrH(%d) = %d, want %d",
+				p, stdoutH, stderrH, stdoutH+stderrH, totalH)
+		}
+	}
+}
+
+func TestUpdateLogViewportSizes_SetsCorrectDimensions(t *testing.T) {
+	m := New()
+	m.width = 200
+	m.height = 101 // totalH = 100
+	m.activePanel = panelJobs
+
+	m.updateLogViewportSizes()
+
+	// jobPanelWidth for width=200: 200*40/100 = 80, capped at 60
+	// rightPanelW = 200 - 60 = 140
+	// viewportWidth = 140 - 4 = 136
+	expectedWidth := 136
+
+	// stderrH = 100 * 20 / 100 = 20, viewportHeight = 20 - 3 = 17
+	// stdoutH = 100 - 20 = 80, viewportHeight = 80 - 3 = 77
+	expectedStderrHeight := 17
+	expectedStdoutHeight := 77
+
+	if m.stderrView.Width != expectedWidth {
+		t.Errorf("stderrView.Width = %d, want %d", m.stderrView.Width, expectedWidth)
+	}
+	if m.stderrView.Height != expectedStderrHeight {
+		t.Errorf("stderrView.Height = %d, want %d", m.stderrView.Height, expectedStderrHeight)
+	}
+	if m.stdoutView.Width != expectedWidth {
+		t.Errorf("stdoutView.Width = %d, want %d", m.stdoutView.Width, expectedWidth)
+	}
+	if m.stdoutView.Height != expectedStdoutHeight {
+		t.Errorf("stdoutView.Height = %d, want %d", m.stdoutView.Height, expectedStdoutHeight)
+	}
+}
+
+func TestUpdateLogViewportSizes_StderrExpands(t *testing.T) {
+	m := New()
+	m.width = 200
+	m.height = 101 // totalH = 100
+
+	// First set to jobs panel (stderr = 20%)
+	m.activePanel = panelJobs
+	m.updateLogViewportSizes()
+	smallStderrHeight := m.stderrView.Height
+	smallStdoutHeight := m.stdoutView.Height
+
+	// Switch to stderr panel (stderr = 80%)
+	m.activePanel = panelStderr
+	m.updateLogViewportSizes()
+	bigStderrHeight := m.stderrView.Height
+	bigStdoutHeight := m.stdoutView.Height
+
+	if bigStderrHeight <= smallStderrHeight {
+		t.Errorf("stderr focused: Height %d should be > unfocused Height %d",
+			bigStderrHeight, smallStderrHeight)
+	}
+	if bigStdoutHeight >= smallStdoutHeight {
+		t.Errorf("stderr focused: stdout Height %d should be < unfocused stdout Height %d",
+			bigStdoutHeight, smallStdoutHeight)
+	}
+}