Fix border width calculation in renderHorizontalEdge

borders.go

@@ -320,8 +320,11 @@
 		width += maxRuneWidth(border.Left)
 	}
 
-	if hasRight && border.Right == "" {
-		border.Right = " "
+	if hasRight {
+		if border.Right == "" {
+			border.Right = " "
+		}
+		width += maxRuneWidth(border.Right)
 	}
 
 	// If corners should be rendered but are set with the empty string, fill them
@@ -435,14 +438,19 @@
 
 	out := strings.Builder{}
 	out.WriteString(left)
-	for i := leftWidth + rightWidth; i < width+rightWidth; {
-		out.WriteRune(runes[j])
+
+	// Fill the middle section
+	middleWidth := width - leftWidth - rightWidth
+	for i := 0; i < middleWidth; {
+		r := runes[j]
+		out.WriteRune(r)
+		i += ansi.StringWidth(string(r))
 		j++
 		if j >= len(runes) {
 			j = 0
 		}
-		i += ansi.StringWidth(string(runes[j]))
 	}
+
 	out.WriteString(right)
 
 	return out.String()

borders_test.go

@@ -1,6 +1,10 @@
 package lipgloss
 
-import "testing"
+import (
+	"testing"
+
+	"github.com/charmbracelet/x/ansi"
+)
 
 func TestStyle_GetBorderSizes(t *testing.T) {
 	tests := []struct {
@@ -94,3 +98,51 @@ func TestStyle_GetBorderSizes(t *testing.T) {
 		})
 	}
 }
+
+func TestRenderHorizontalEdgeFix(t *testing.T) {
+	tests := []struct {
+		name     string
+		left     string
+		middle   string
+		right    string
+		width    int
+		expected int // expected display width
+	}{
+		{
+			name:     "standard border 20 width",
+			left:     "┌",
+			middle:   "─",
+			right:    "┐",
+			width:    20,
+			expected: 20,
+		},
+		{
+			name:     "standard border 45 width",
+			left:     "╭",
+			middle:   "─",
+			right:    "╮",
+			width:    45,
+			expected: 45,
+		},
+		{
+			name:     "ascii border 30 width",
+			left:     "+",
+			middle:   "-",
+			right:    "+",
+			width:    30,
+			expected: 30,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := renderHorizontalEdge(tt.left, tt.middle, tt.right, tt.width)
+			actual := ansi.StringWidth(result)
+
+			if actual != tt.expected {
+				t.Errorf("renderHorizontalEdge() width = %d, expected %d", actual, tt.expected)
+				t.Errorf("Result: %q", result)
+			}
+		})
+	}
+}