fix(textarea): don't hardcode line number gutter width

textarea/textarea.go

@@ -4,6 +4,7 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"image/color"
+	"math"
 	"strconv"
 	"strings"
 	"time"
@@ -988,6 +989,9 @@ func (m *Model) SetWidth(w int) {
 	if m.promptFunc == nil {
 		// XXX: This should account for a styled prompt and use lipglosss.Width
 		// instead of uniseg.StringWidth.
+		//
+		// XXX: Do we even need this or can we calculate the prompt width
+		// at render time?
 		m.promptWidth = uniseg.StringWidth(m.Prompt)
 	}
 
@@ -999,9 +1003,10 @@ func (m *Model) SetWidth(w int) {
 
 	// Add line number width to reserved inner width.
 	if m.ShowLineNumbers {
-		// XXX: this should almost certainly not be hardcoded.
-		const lnWidth = 4 // Up to 3 digits for line number plus 1 margin.
-		reservedInner += lnWidth
+		const gap = 1
+
+		// Number of digits plus 1 cell for the margin.
+		reservedInner += numDigits(m.MaxWidth) + gap
 	}
 
 	// Input width must be at least one more than the reserved inner and outer
@@ -1610,6 +1615,11 @@ func repeatSpaces(n int) []rune {
 	return []rune(strings.Repeat(string(' '), n))
 }
 
+// numDigits returns the number of digits in an integer.
+func numDigits(n int) int {
+	return int(math.Floor(math.Log10(float64(n)) + 1))
+}
+
 func clamp(v, low, high int) int {
 	if high < low {
 		low, high = high, low