Keep terminal textbox cursor on rune boundaries

fix.txt

@@ -238,7 +238,7 @@ Priority 3: configuration, UI, and binding hardening
     fails, hiding startup failure. Return that error. Also stop/join the
     PollEvent goroutine on UI shutdown and make Close nonblocking/idempotent.
 
-31. Text UI is not Unicode-safe and timestamp parsing is fragile
+[x] 31. Text UI is not Unicode-safe and timestamp parsing is fragile
     Files: uiterm/textbox.go, uiterm/textview.go
     Textbox cursor positions are byte offsets but editing/display iterates
     runes, so non-ASCII input can be split into invalid UTF-8. Textview assumes

uiterm/textbox.go

@@ -2,7 +2,7 @@ package uiterm
 
 import (
 	"strings"
-	//	"unicode/utf8"
+	"unicode/utf8"
 
 	"github.com/nsf/termbox-go"
 )
@@ -41,6 +41,9 @@ func (t *Textbox) uiSetBounds(x0, y0, x1, y1 int) {
 }
 
 func (t *Textbox) uiDraw() {
+	if t.ui == nil {
+		return
+	}
 	t.ui.beginDraw()
 	defer t.ui.endDraw()
 
@@ -51,6 +54,9 @@ func (t *Textbox) uiDraw() {
 	if t.pos > len(t.Text) {
 		t.pos = len(t.Text)
 	}
+	for t.pos > 0 && t.pos < len(t.Text) && !utf8.RuneStart(t.Text[t.pos]) {
+		t.pos--
+	}
 	for y := t.y0; y < t.y1; y++ {
 		for x := t.x0; x < t.x1; x++ {
 			var chr rune
@@ -95,10 +101,16 @@ func (t *Textbox) uiKeyEvent(key Key) {
 		t.pos = len(t.Text)
 		redraw = true
 	case KeyArrowLeft:
-		t.pos -= 1
+		if t.pos > 0 {
+			_, size := utf8.DecodeLastRuneInString(t.Text[:t.pos])
+			t.pos -= size
+		}
 		redraw = true
 	case KeyArrowRight:
-		t.pos += 1
+		if t.pos < len(t.Text) {
+			_, size := utf8.DecodeRuneInString(t.Text[t.pos:])
+			t.pos += size
+		}
 		redraw = true
 	case KeyCtrlC:
 		t.Text = ""
@@ -119,12 +131,12 @@ func (t *Textbox) uiKeyEvent(key Key) {
 		redraw = t.handleHistoryKey(key)
 	case KeySpace:
 		t.uiCharacterEvent(' ')
-	case KeyBackspace:
-	case KeyBackspace2:
+	case KeyBackspace, KeyBackspace2:
 		if len(t.Text) > 0 {
 			if t.pos > 0 {
-				t.Text = t.Text[:t.pos-1] + t.Text[t.pos:]
-				t.pos -= 1
+				_, size := utf8.DecodeLastRuneInString(t.Text[:t.pos])
+				t.Text = t.Text[:t.pos-size] + t.Text[t.pos:]
+				t.pos -= size
 			}
 		}
 		//			if r, size := utf8.DecodeLastRuneInString(t.Text); r != utf8.RuneError {

uiterm/textbox_test.go

@@ -2,6 +2,24 @@ package uiterm
 
 import "testing"
 
+// Regression: byte-based cursor movement split UTF-8 input, producing invalid
+// text when deleting or inserting beside a non-ASCII character.
+func TestTextboxEditsAtRuneBoundaries(t *testing.T) {
+	textbox := Textbox{Text: "aé", pos: len("aé")}
+	textbox.uiKeyEvent(KeyArrowLeft)
+	if textbox.pos != 1 {
+		t.Fatalf("cursor = %d, want rune boundary 1", textbox.pos)
+	}
+	textbox.uiKeyEvent(KeyBackspace)
+	if textbox.Text != "é" || textbox.pos != 0 {
+		t.Fatalf("after delete: %q at %d", textbox.Text, textbox.pos)
+	}
+	textbox.uiCharacterEvent('ß')
+	if textbox.Text != "ßé" {
+		t.Fatalf("insert produced %q", textbox.Text)
+	}
+}
+
 func TestTextboxHistoryNavigatesSubmittedText(t *testing.T) {
 	t.Parallel()