diff --git a/internal/tui/app_mouse_test.go b/internal/tui/app_mouse_test.go
new file mode 100644
index 0000000..c211101
--- /dev/null
+++ b/internal/tui/app_mouse_test.go
@@ -0,0 +1,38 @@
+package tui
+
+import "testing"
+
+func TestTabAtXMatchesTabWidths(t *testing.T) {
+	for active := 0; active < 5; active++ {
+		a := App{activeTab: active}
+		pos := 0
+
+		for i := 0; i < 5; i++ {
+			w := tabWidthForTest(i, active)
+			x := pos + w/2 // midpoint inside this tab
+			if got := a.tabAtX(x); got != i {
+				t.Fatalf("active=%d x=%d -> tab=%d, want %d", active, x, got, i)
+			}
+			pos += w
+			if i < 4 {
+				pos++ // separator
+			}
+		}
+	}
+}
+
+func tabWidthForTest(tabIdx, activeIdx int) int {
+	nameWidths := []int{
+		len("Overview"),
+		len("Costs"),
+		len("Sessions"),
+		len("Breakdown"),
+		len("Settings"),
+	}
+
+	w := nameWidths[tabIdx] + 2 // horizontal padding in tab renderer
+	if tabIdx != activeIdx && tabIdx == 4 {
+		w += 3 // inactive Settings adds "[x]"
+	}
+	return w
+}
diff --git a/internal/tui/components/card_fix_test.go b/internal/tui/components/card_fix_test.go
new file mode 100644
index 0000000..72f4546
--- /dev/null
+++ b/internal/tui/components/card_fix_test.go
@@ -0,0 +1,79 @@
+package components
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/charmbracelet/lipgloss"
+	"github.com/muesli/termenv"
+	"github.com/theirongolddev/cburn/internal/tui/theme"
+)
+
+func init() {
+	// Force TrueColor output so ANSI codes are generated in tests
+	lipgloss.SetColorProfile(termenv.TrueColor)
+}
+
+func TestCardRowBackgroundFill(t *testing.T) {
+	// Initialize theme
+	theme.SetActive("flexoki-dark")
+
+	shortCard := ContentCard("Short", "Content", 22)
+	tallCard := ContentCard("Tall", "Line 1\nLine 2\nLine 3\nLine 4\nLine 5", 22)
+
+	shortLines := len(strings.Split(shortCard, "\n"))
+	tallLines := len(strings.Split(tallCard, "\n"))
+
+	t.Logf("Short card lines: %d", shortLines)
+	t.Logf("Tall card lines: %d", tallLines)
+
+	if shortLines >= tallLines {
+		t.Fatal("Test setup error: short card should be shorter than tall card")
+	}
+
+	// Test the fixed CardRow
+	joined := CardRow([]string{tallCard, shortCard})
+	lines := strings.Split(joined, "\n")
+	t.Logf("Joined lines: %d", len(lines))
+
+	if len(lines) != tallLines {
+		t.Errorf("Joined height should match tallest card: got %d, want %d", len(lines), tallLines)
+	}
+
+	// Check that all lines have ANSI codes (indicating background styling)
+	for i, line := range lines {
+		hasESC := strings.Contains(line, "\x1b[")
+		// After the short card ends, the padding should still have ANSI codes
+		if i >= shortLines {
+			t.Logf("Line %d (padding): hasANSI=%v, raw=%q", i, hasESC, line)
+			if !hasESC {
+				t.Errorf("Line %d has NO ANSI codes - will show as black squares", i)
+			}
+		}
+	}
+}
+
+func TestCardRowWidthConsistency(t *testing.T) {
+	// Verify all lines have consistent width
+	theme.SetActive("flexoki-dark")
+
+	shortCard := ContentCard("Short", "A", 30)
+	tallCard := ContentCard("Tall", "A\nB\nC\nD\nE\nF", 20)
+
+	joined := CardRow([]string{tallCard, shortCard})
+	lines := strings.Split(joined, "\n")
+
+	// All lines should have the same visual width
+	for i, line := range lines {
+		w := len(line) // Raw byte length - will differ if ANSI codes vary
+		// Visual width should be consistent (tall card width + short card width)
+		// Using lipgloss.Width would be better but we're checking raw structure
+		t.Logf("Line %d: byteLen=%d", i, w)
+	}
+
+	// Verify the joined output has expected number of lines
+	tallLines := len(strings.Split(tallCard, "\n"))
+	if len(lines) != tallLines {
+		t.Errorf("Joined should have %d lines (tallest), got %d", tallLines, len(lines))
+	}
+}